Soldering method

ABSTRACT

In a soldering method for soldering an electronic component including a palladium or palladium alloy layer formed on a surface of the electronic component and also including a soldering lead terminal onto a printed wiring board including a soldering land and plated through hole, a solder layer containing tin and zinc as main components is formed on the surfaces of the land through hole by a HAL treatment. The lead terminal is inserted and mounted in the through hole. The printed wiring board is brought into contact with jet flows of a solder containing tin and zinc as the main components to thereby supply a solder to the land and through hole.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a division of application Ser. No. 10/991,401, filedNov. 19, 2004, which is a division of application Ser. No. 10/339,643,filed Jan. 10, 2003, now U.S. Pat. No. 6,902,102, issued Jun. 7, 2005,and based on Japanese Patent Application No. 2002-004185, filed Jan. 11,2002; Japanese Patent Application No. 2002-067870, filed Mar. 13, 2002;and Japanese Patent Application No. 2002-282704, filed Sep. 27, 2002, byKazuhiko Tanabe, Hiroaki Terada, Masahiro Sugiura, Tetsuharu Mizutni,Keiichiro Imamura, and Takashi Tanaka, all of which are incorporatedherein by reference in their entirety. This application claims onlysubject matter disclosed in the parent application and thereforepresents no new matter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to soldering method of an electroniccomponent of a soldered lead terminal plated with a lead-free metal, anda solder joint member.

Moreover, the present invention relates to a soldering method forsoldering a printed wiring board including portions to be soldered inopposite surfaces, comprising: reflow-soldering one surface; andsubsequently flow-soldering the other surface. The present inventionparticularly relates to a soldering method for performing a solderingoperation in the above-described procedure, in which a fillet peel(circuit disconnection) is prevented from being generated in thereflow-soldered portion upon the flow-soldering.

Further, the present invention relates to a flow soldering method by atin-zinc solder (solder mainly containing tin and zinc) which issuperior in solder wetting/spreading and solder wetting/rising in aplated through hole even at a low temperature.

2. Description of the Related Art

(First Related Art)

As a substitute for plating by a lead-containing solder, a palladium ora palladium alloy layer has been formed on a lead frame surface of asemiconductor component. However, palladium has a problem thatwettability is bad with respect to the tin-lead solder heretofore used,or potential candidates of a lead-free solder, such as atin-silver-copper based solder and tin-copper based solder. There isalso a lift-off problem that the solder floats from a copper land of theprinted wiring board. Such related art and problems will be describedhereinafter in detail.

As described above, as a technique for the lead-free electroniccomponent, a method of forming the palladium or palladium alloy layer onthe surface of the lead frame of the semiconductor component by platinghas been performed. This is because palladium is superior in wettabilitywith respect to a bonding wire of a wax material (silver wax) or goldduring die bonding.

It is to be noted that rust is often generated in the lead frame only ofthe palladium or palladium alloy layer. Therefore, a nickel layer isgenerally formed as an underlayer.

FIG. 1 is a diagram showing a section of the lead frame (soldering lead)of the semiconductor component. For ease of seeing, a rather thickplated layer is shown.

That is, in a soldering lead 30, for example, the surface of a leadframe material such as an iron-nickel alloy 31 is once coated with theunderlayer including a nickel layer 32, and further coated with an upperlayer of a palladium layer 33 (or palladium alloy layer).

In order to mount electronic components such as the semiconductorcomponent on the printed wiring board and actually solder the componentby a flow soldering method, a method comprises: inserting the solderinglead terminal in the through hole of the printed wiring board; andbringing a soldering land formed around the through hole, and thesoldering lead terminal into contact with a jet flow of a molten solderto solder the component. Under such a circumstance, a through hole isplated with copper to form the plated through hole in order to performhigh-reliability soldering/mounting.

FIG. 2 is a longitudinal section view of the through hole, showing thatthe soldering lead terminal of the semiconductor component is insertedin the plated through hole of the printed wiring board.

Specifically, soldering lands 41 are disposed on the lower and uppersurfaces of a printed wiring board 40, and these lands 41 are connectedto each other via a plated through hole 42. A soldering lead terminal 43of a semiconductor device is inserted in the through hole 42, and aportion to be soldered 44 is constituted between the lands 41 and platedthrough hole 42 and the soldering lead terminal 43.

In this condition, the jet flow of the molten solder is brought intocontact with the lower surface of the printed wiring board 40 from belowin FIG. 2, that is, the portion to be soldered 44 on the lower surfaceof the board to perform the flow soldering.

During the soldering/mounting, it is necessary to supply the solderhaving a sufficiently molten state to the portion to be soldered 44 sothat satisfactory wettability is obtained. In other words, this isnecessary for satisfying satisfactory electric connection and sufficientmechanical strength/connection of the portion to be soldered 44.Moreover, the printed wiring board 40 including the plated through hole42 must be sufficiently filled with the solder in the through hole 42and be wetted up.

In the related art, the electronic component is soldered onto theprinted wiring board, liquid flux is generally applied in order tosecure soldering property. This liquid flux is applied by methods suchas immersing, brushing, spraying, and foaming. A post flux for use in anelectronic industry is generally constituted of a main componentobtained by blending and dissolving rosin as a base resin andhydrohalide of amine as an activator in a lower alcohol-based organicsolvent such as isopropyl alcohol.

However, since the temperature of the lead frame rises in a die bondingor wire bonding step during the manufacturing of the semiconductorcomponent, even palladium or palladium alloy superior in a bondingproperty has a problem of deterioration of solder wettability of thelead frame during the soldering/mounting onto the printed wiring board.That is, the solder wettability of the soldering lead terminal of thesemiconductor component is deteriorated.

FIG. 3 is a longitudinal section view of the through hole, showing asoldering state of the soldering lead terminal whose wettability isdeteriorated.

Specifically, the deterioration of the wettability of a soldering leadterminal 53 prevents the wetting-up of the solder into a plated throughhole 52, and a soldering land 51 disposed on the upper surface of aprinted wiring board 50 is not sufficiently wetted up. Such wetting-updefect of a solder 54 is similarly generated in the tin-lead solderheretofore used, or the potential candidates of the lead-free soldersuch as the tin-silver-copper based solder and tin-copper based solder.

On the other hand, as the technique for the lead-free solder, thetin-silver-copper based solder is the potential candidate. This solderis used to prepare the soldering lead terminal of the electroniccomponent subjected to various types of plating, and the terminal isinserted in the through hole plated with the pre-flux as the potentialcandidate for a printed wiring board surface treatment, which does notcontain lead. When the tin-silver-copper based solder is used to solderthe component, a disadvantage referred to as lift-off (bonding defect)is known to be generated. That is, the solder floats from the copperland of the printed wiring board. FIGS. 4A, 4B are bird's-eye views ofthe lift-off generated in the lead-free solder.

As described above, the solder in the molten state is not sufficientlyfilled or wetted into the plated through hole, or the lift-off occurs.Then, when the printed wiring board is mounted on the electronicapparatus and used, because of applied stresses (e.g., temperaturechange, vibration, acceleration, deformation by deflection of theprinted wiring board, and the like) or changes with time, the solderinglands, that is, printed wirings disposed on the lower and upper surfacesof the printed wiring board are disconnected each other. Alternatively,there is a problem that the solder bonding is detached and thedisconnection easily occurs. In other words, reliability of theelectronic apparatus on which the printed wiring board is mounted isremarkably degraded.

(Second Related Art)

FIG. 5 is an explanatory view of an example of a soldering procedure inwhich one surface of a printed wiring board 60 is reflow-soldered andthe other surface is flow-soldered by a related-art method. It is to benoted here that this related-art example shows an example of the printedwiring board 60 including a plated through hole 61 and encirclednumerals 1 to 9 denote step numbers.

Specifically, at first in step 1, an adhesive 63 for bonding anelectronic component 12 (e.g., SMD such as a chip component) to beflow-soldered is applied, and subsequently in step 2 the correspondingelectronic component 12 is mounted. Thereafter, in step 3 the adhesive63 applied in the step 1 is hardened, and the electronic component 12 isfixed onto a land 64 (circuit pattern).

Next in step 4, the printed wiring board 60 is reversed, and solders 65such as cream solders are supplied beforehand onto the lands 64 on whichsoldering terminals of electronic components 62, 66 to bereflow-soldered are laid. Subsequently, in step 5, the electroniccomponents 62, 66 such as the chip component and QFP IC are mounted.

Thereafter, in step 6, the reflow soldering is performed by a reflowsoldering apparatus. Specifically, the solder 65 supplied in the step 4is heated and molten, and portions to be soldered of the electroniccomponents 62, 66 such as the chip component and QFP IC betweensoldering terminals 67 and lands 64 are soldered.

After the reflow soldering is completed, in step 7, for example, leadterminals 69 of an insertion type electronic component 68 (hereinafterreferred to as an insertion component) such as a connector is insertedin the through hole 61 on the side of the reflow soldered surface of theprinted wiring board. In step 8 the flow soldering is carried out by aflow soldering apparatus.

Specifically, the portion to be soldered formed between the land 64 andthe electronic component 62 fixed on the land 64 in the step 3, and theportion to be soldered formed between the land 64 including the platedthrough hole 61 and the lead terminals 69 of the insertion component 68inserted in the through hole 61 in the step 7 are brought into contactwith the jet flow of the solder in the molten state. The solder issupplied to these portions to be soldered, and the flow soldering isperformed.

Herein, it is general to apply the liquid flux in order to secure thesoldering property upon the flow soldering of the electronic component12 or insertion component 68 onto the printed wiring board 60. Suchliquid flux is applied by the methods such as immersing, brushing,spraying, and foaming. The post flux for use in the electronic industryis generally constituted of the main component obtained by blending anddissolving rosin as the base resin and hydrohalide of amine as theactivator in the lower alcohol-based organic solvent such as isopropylalcohol.

By the above-described procedure, the printed wiring board 60 is cooledas in step 9 of FIG. 5, the soldering of the portions to be soldered inthe opposite surfaces of the board is completed, and fillets 600 of thesolder are formed on the portions to be soldered of the SMD (electroniccomponent 12) and insertion component 68.

In this event, in case where the land 64 disposed on one surface of theprinted wiring board 60 is connected to the land 64 disposed on theother surface via the through hole 61, in the flow soldering step 8, thesolder 65 needs to wet up the inside of the plated through hole 61 andwet-spread over the opposite lands 64. Moreover, in case where the leadterminals 69 of the insertion component 68 are inserted in the platedthrough holes 61, as shown in the step 9, the solder 65 needs towet-spread over the opposite lands 64 to form the fillets 600.

This prevents the plated through hole 61 from being cracked and causingcircuit disconnection by stresses such as heat stress, acceleration, andvibration upon the mounting of the printed wiring board 60 onto theelectronic apparatus. Therefore, reliability of the electronic apparatusoperating by the printed wiring board 60 is prevented from beingdamaged.

Moreover, it is most important to prevent the solder of the portionreflow-soldered in the step 6 from being molten again in the step 8 ofthe flow soldering. That is, the temperature of the reflow-solderedportion rises by heat conducted from the jet flow of the solder whichhas been brought into contact with the printed wiring board 60 upon theflow soldering.

When the solder of the reflow-soldered portion is molten again duringthe flow soldering, the position of the reflow-soldered electroniccomponent 62 moves during the flow soldering and a soldering defect isgenerated. In the extreme, the electronic component 62 is detached andmoved from the land 64 constituting the portion to be soldered, and acircuit function defect is caused. Even when the re-melting simplyoccurs, solder wet defect occurs and soldering strength drops.

To solve the problem, various soldering methods have been proposed.

For example, one example is described in Japanese Patent ApplicationLaid-Open No. 2001-358456 (hereinafter referred to also as PatentDocument 1). That is, in this related-art example, a tin-silver-bismuthbased lead-free solder and tin-zinc-bismuth based lead-free solder areused. In this technique, “a melting range of each solder is allowed toshift and thereby disengagement or connection strength drop of thecomponent connected by a second-surface reflow or first-surface reflowupon the flow soldering are suppressed” (see paragraph [0009] of therelated-art example of Patent Document 1).

In the technique disclosed in the related-art example, bismuth is mainlyused as a melting point adjustment agent, and a bismuth content in thesolder is controlled to adjust the melting range of the solder. However,when the solder contains bismuth, the solder of the portion to besoldered is easily embrittled. There is a problem that solderingstrength rapidly and easily drops with respect to a cycle stress givento the portion to be soldered. In other words, there is no tenacity.

Further, in the related-art example, “phenomenon in which the componentpeels off from an electrode on a substrate together with the solderdepending on the type of the component” (see paragraph [0005] of therelated-art example) is described. However, a mechanism by which thephenomenon occurs is not clarified. Therefore, the problem is not solvedby a theoretically appropriate technique. That is, bismuth is includedin the lead-free solder, the melting point is thereby forced to drop,and an influence of temperature rise during the flow soldering isminimized if possible in the technique.

The present inventors have noted that “the phenomenon in which thecomponent peels from the electrode on the substrate together with thesolder,” that is, the fillet peel from the land occurs only with respectto the electronic component coated with the tin-lead solder orincluding, for example, the plated soldering terminal during thesoldering using a tin-silver-x (another element) based lead-free solder,and have observed the peeled portion using an electronic microscope. Asa result, segregation of lead has been recognized in the peeled portion.Specifically, it has been clarified that a micro ternary eutectic alloycomposition of tin-silver-lead is formed in an interface of the land andfillet and the melting point is as low as 178° C. (see “ExperimentalConsideration concerning SMT Fillet Peel of Lead-Free Solder” KazuhikoTanabe, Yu Saito, Article Name: the 15th Electronics Mounting AcademicLecture General Assembly Collected Papers).

FIGS. 6A and 6B are diagrams showing an end surface of the portion to besoldered showing the fillet peel, FIG. 6A is an explanatory view of anormal soldered state, and FIG. 6B is an explanatory view of the stateof the peeled fillets 600. In the drawing, a peeled portion 70 is shownin an exaggerated manner for ease of seeing. Thus, the fillet peel isgenerated in a peeled manner from the interface of the land 64 andsolder.

It is to be noted that in the above-described related-art example“three-elements based solders such as Sn—Ag—Bi and Sn—Zn—Bi basedsolders are noted in a close-up manner as potential candidates of aPb-free solder alloy replacing the Sn—Pb based solder.” “Sn-9 weight %Zn (melting point 199° C.) has a substantially appropriate meltingpoint, but the solder surface is remarkably oxidized in the soldering inthe atmosphere, and the solder is not easily used” (see paragraph [0003]of a first related-art example). This is set aside in a boundary pointof the scope of the present invention.

Particularly, at a filing time of this related-art example, the flowsoldering technique of the printed wiring board using the tin-9 zincsolder (tin-9 wt % zinc solder: the numeral before the element similarlydenotes weight % hereinafter) had not been established yet. The flowsoldering technique of the printed wiring board using the tin-9 zincsolder was thereafter published by Japanese Patent Application Laid-OpenNo. 2001-293559 as Patent Document 2, and technical establishment isknown.

(Third Related Art)

Diffusion of lead toxicity into natural environments and influencethereof onto human bodies have raised problems, and the soldering of theelectronic apparatus and printed wiring board has been performed by thelead-free solder without using the lead. However, most of presently usedlead-free solders are lead-free solders, whose melting point is high atabout 220° C. An optimum soldering temperature is higher than thesoldering temperature (about 240° C. to 250° C.) of the tin-lead solderwhich has heretofore been used by about 10° C. to 15° C. and is in arange of about 250° C. to 255° C.

To flow-solder the printed wiring board on which the electroniccomponent is mounted, namely, a work to be soldered, a heat stress isimposed onto the printed wiring board and electronic component.

Specifically, it is necessary to bring the portion to be soldered, thatis, the surface to be soldered of the printed wiring board into contactwith the solder having the molten state and to solder the portion.

Moreover, the heat stress of the presently used lead-free solder havingthe high melting point becomes larger than before, and therefore aproblem is that life of the electronic apparatus becomes shorter thanbefore. As a countermeasure, development for enhancing heat resistanceof the electronic component and development for realizing the flowsoldering with a low-temperature solder have been performed.

The heat stress has a large influence on the life of the electroniccomponent. Specifically, when the flow soldering is performed with thelow-temperature solder, the electronic apparatus having a long life canbe realized. Additionally, if the solder having an optimum solderingtemperature lower than that of the tin-lead solder heretofore used isemployed, the electronic apparatus having a longer life than before canbe realized.

As described above, the temperature of the solder during the flowsoldering appears as a difference of reliability which cannot apparentlybe judged from the electronic apparatus. Therefore, as an importantindex in supplying the high-reliability electronic component to a user,a flow soldering technique using the solder having a low optimumsoldering temperature has been noted.

The melting point of the tin-zinc solder (e.g., the melting point of thetin-9 zinc solder) is about 199° C. and is low as compared with themelting point of another lead-free solder (e.g., about 220° C.). Thereis an advantage that the heat stress imposed onto the printed wiringboard or the electronic component can be reduced during the flowsoldering. However, it is regarded as impossible to flow-solder theprinted wiring board using the tin-zinc solder, because solderingproperties such as the wettability are not good.

Already in Japanese Patent Application No. 2002-4185, the presentinventors have filed a patent application of a technique comprising:subjecting the land to be soldered or plated through hole of the printedwiring board to leveler treatments such as a hot air leveler (HAL)treatment by the tin-zinc solder to form a solder layer containing tinand zinc as main components on the surface of the board. Thereby, theelectronic component in which the palladium or palladium alloy layer isformed on the surface of the soldering lead terminal of the electroniccomponent can be soldered with superior wettability, and connectionreliability of the portion to be soldered is also high.

Thereby, it has become possible to perform the flow soldering with thesolder which has a temperature similar to or lower than the conventionaltemperature. The palladium or palladium alloy layer has been used for alead-free member replacing the tin-zinc solder layer heretofore broadlyused.

Herein, it is to be noted that the HAL treatment is a treatmentcomprising: immersing the printed wiring board into the solder or metalhaving the molten state; thereafter drawing the printed wiring board outof the solder in the molten state; and spraying air or inactive gas tothe board to level the board.

For a printed wiring board on which a large number of various electroniccomponents are mounted and which includes a large number of portions tobe soldered, to collectively flow-solder the respective portions to besoldered, major factors for influencing the soldering properties such asthe wettability and wet-up property of the solder are controlled.Thereby, the influence of other sub factors onto the solderingproperties can be ignored. This enhances and uniforms solderingqualities of a large number of portions to be soldered. Additionally,this is necessary for enhancing and stabilizing the soldering qualitiesof the printed wiring boards produced in mass.

Therefore, in the flow soldering method using the tin-zinc solder, thepresent inventors have specified major factors in the printed wiringboard, for enhancing the wettability and wet-up property of the tin-zincsolder in the land to be soldered and plated through hole in the printedwiring board. It has been regarded as a technical problem to clarifyconditions for performing the flow soldering method using the tin-zincsolder with the stable soldering quality, and the flow soldering methodhas been developed.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide asoldering method in which soldering/mounting superior in solderwettability is possible, and a solder joint member superior inwettability in order to solve the problem of the second related art,

Moreover, another object of the present invention is to realize aprinted wiring board superior in overall reliability including apackaging technique of a semiconductor component and to thereby realizean electronic apparatus having a high reliability as the whole apparatusincluding the electronic components (first object).

Further, the present invention has been developed in consideration ofthe problem of the second related art, and an object thereof is tosecurely prevent land peel during use of a lead-free solder and makepossible the soldering/packaging of a printed wiring board superior insoldering quality and reliability (second object).

Additionally, the present invention has been developed in considerationof the problem of the third related art, and an object thereof is toprovide a flow soldering method using a tin-zinc solder, in whichconditions of a printed wiring board superior in wettability of a solderin a portion to be soldered and wetting-up of the solder of a platedthrough hole are specified and thereby a low-temperature solder is usedto make possible the soldering/packaging of the printed wiring boardsuperior in the soldering quality and reliability (third object).

(First Aspect of the Invention)

To achieve the first object, according to the present invention, thereis provided a soldering method for soldering an electronic componentincluding a palladium or palladium alloy layer formed on the surfacethereof and also including a soldering lead terminal to a printed wiringboard including a soldering land and plated through hole, the methodcomprising the steps of: forming a solder layer containing tin and zincas main components on the land and the surface of the through hole by aHAL treatment; inserting and mounting the lead terminal in the throughhole; bringing the printed wiring board into contact with a jet flow ofa solder containing tin and zinc as the main components; and therebysupplying the solder to the land and through hole.

Here, as described above, the HAL treatment is a treatment for sprayingheated air to the substrate dipped in a bath of molten solder to levelthe substrate in a method of coating a substrate with the solder andsoldering the electronic component on the coating in order to enhance asoldering property in the electronic circuit substrate.

Under this circumstance, when the printed wiring board is brought intocontact with the jet flow of the solder, the solder wets the leadterminal and wets upwards to reach the upper surface of the land via theinside of the through hole.

Here, a flux for use in soldering the lead terminal with the solderpreferably contains an acrylic acid adduct of rosin and an activatorincluding a sarcosine framework.

Moreover, according to the present invention, there is provided asoldering method for soldering an electronic component including asoldering lead terminal plated as desired to a printed wiring boardincluding a plated through hole, comprising the steps of: forming asolder layer containing tin and zinc as main components on the surfaceof the through hole by a HAL treatment; inserting and mounting the leadterminal in the through hole; bringing the printed wiring board intocontact with a jet flow of a solder containing tin and zinc as the maincomponents; and thereby supplying the solder to the through hole so thatlift-off is reduced or prevented.

Under this circumstance, when the printed wiring board is brought intocontact with the jet flow of the solder, the solder wets the leadterminal and wets upwards via the inside of the through hole.

Here, the flux for use in soldering the lead terminal with the solderpreferably contains the acrylic acid adduct of rosin and the activatorincluding the sarcosine framework.

Moreover, according to the present invention, there is provided a solderjoint member for soldering a semiconductor component to a printed wiringboard. The semiconductor component includes a soldering lead terminalobtained by forming a palladium or palladium alloy layer on the surfaceof the terminal, the printed wiring board includes a soldering land orsoldering through hole formed of copper, and the semiconductor componentis joined to the land or through hole via a solder layer containing tinand zinc as main components.

Here, the semiconductor component includes, for example, a lead frame.

Further, the flux for use in soldering the lead terminal with the solderpreferably contains the acrylic acid adduct of rosin and the activatorincluding the sarcosine framework.

Additionally, according to the present invention, there is provided asolder joint member for soldering an electronic component to a printedwiring board. The electronic component includes a soldering leadterminal plated as desired, and the printed wiring board includes asoldering through hole including a first solder layer containing tin andzinc as main components formed on the surface of the hole by a HALtreatment. The lead terminal is inserted in the through hole andmounted, and includes a second solder layer which mainly contains tinand zinc as the main components and which is formed by bringing theprinted wiring board into contact with a jet flow of a solder containingtin and zinc as the main components. The electronic component is joinedto the through hole via the second solder layer.

Under this constitution, the joining by the second solder layer isperformed so as to reduce or prevent the lift-off.

Moreover, the flux for use in soldering the lead terminal with thesolder preferably contains the acrylic acid adduct of rosin and theactivator including the sarcosine framework.

The soldering method and solder joint member according to the presentinvention is characterized in a constitution in which the soldersuperior in wettability with respect to palladium or palladium alloy canbe used to form a superior soldering state between the soldering landand through hole formed of copper.

(1) That is, there is provided a soldering method comprising: insertingand mounting a soldering lead terminal of an electronic componentincluding a palladium or palladium alloy layer formed on the surfacethereof in a through hole of a printed wiring board which includes asoldering land and plated through hole and in which a solder layercontaining tin and zinc as main components is formed on the solderingland and the surface of the plated through hole by a HAL treatment;bringing the printed wiring board into contact with a jet flow of asolder containing tin and zinc as the main components; and therebysupplying the solder to the soldering land and plated through hole tosolder the electronic component.

Palladium and palladium alloy indicate a satisfactory wettability to thesoldering which contains tin and zinc as the main components. Moreover,the layer of the tin-zinc solder which contains tin and zinc as the maincomponents and which sufficiently wets is formed on the soldering landand the surface of the plated through hole formed of copper by the HALtreatment.

Therefore, the soldering land and plated through hole on which thesolder layer containing tin and zinc as the main components is formed,the plated through hole, and the soldering lead terminal of theelectronic component which is inserted in the through hole and whichincludes the palladium or palladium alloy layer formed on the surface ofthe terminal are brought into contact with the jet flow of the solderwhich contains tin and zinc as the main components. Then, the solderwets the soldering lead terminal and sets upwards the inside of theplated through hole, and the satisfactory soldered state superior inwettability can be obtained.

(2) Further, there is provided a solder joint member constituted to joina semiconductor component which includes a lead frame and in which apalladium or palladium alloy layer is formed on the surface of asoldering lead terminal of the semiconductor component to a solderingland or soldering through hole of a printed wiring board formed ofcopper via a solder containing tin and zinc as main components.

The solder containing tin and zinc as the main components hasappropriate hardness and superior elongation characteristic. Forexample, a high-temperature elongation in 125° C. indicates 66% for aconventional solder containing tin and 37 weight % of lead, indicates anequal or more value 67% for a solder containing tin and 9 weight % ofzinc, and only indicates 12% for a solder containing tin, 3 weight % ofsilver, and 0.5 weight % of copper. That is, with the joining by thesolder containing tin and zinc as the main components, a tough solderjoint member can be obtained.

Moreover, the solder containing tin and zinc as the main components hasa superior wettability with respect to palladium or palladium alloy andalso with respect to copper.

(3) Further, it has been found that the acrylic acid adduct of rosin andthe activator including the sarcosine framework are used in a fluxcomposition and thereby further satisfactory solder wettability isachieved. The present invention has been completed.

Therefore, according to the solder joint member of the presentinvention, the solder joint is tenacious in a high-reliability lead-freesemiconductor component in which palladium is used in the plating of thelead frame, and in the soldering/mounting of the lead-free semiconductorcomponent onto the printed wiring board. Not only mechanical connectionbut also electric connection are superior. From the solder joint memberin the soldering/mounting of these electronic components onto theprinted wiring board, an electronic apparatus remarkably superior inreliability can be realized.

(Second Aspect of the Invention)

To achieve the second object, according to the present invention, thereis provided a soldering method for soldering with respect to a printedwiring board having portions to be soldered on opposite surfaces,comprising the steps of: reflow-soldering an electronic component to onesurface of the printed wiring board by a lead-free solder containing atleast tin, silver, and copper; subsequently controlling the soldering ofthe portion to be soldered formed on the other surface of the printedwiring board by a tin-9 zinc solder in a temperature range of 220° C. to230° C.; and holding the temperature of the reflow-soldered portion tobe soldered at 178° C. or less to perform flow soldering.

Here, the electronic component includes a soldering terminal coated witha tin-lead solder.

Further, a plated through hole is formed in the printed wiring board,and further the printed wiring board is subjected to HAL treatment by atin-zinc solder. Here, the HAL treatment is a treatment for sprayingheated gas (air) to the substrate dipped in a bath of molten solder tolevel the substrate in a method of coating a substrate with the solderand soldering the electronic component on the coating in order toenhance a soldering property in the electronic circuit substrate.

Moreover, the flux for use in the flow soldering preferably contains theacrylic acid adduct of rosin and the activator including the sarcosineframework.

In this case, the solder does not contain bismuth.

The soldering method of the present invention is characterized in thatthe flow soldering is performed so as to prevent the temperature of theportion to be soldered on a reflow-soldered surface from exceeding 178°C. as a melting point of a ternary eutectic alloy of tin-silver-lead.

(1) Specifically, there is provided a soldering method comprising:reflow-soldering an electronic component including a soldering terminalcoated with a tin-lead solder to one surface of a printed wiring boardby a lead-free solder containing at least tin, silver, and copper;subsequently controlling the solder of the portion to be soldered formedon the other surface of the printed wiring board by a tin-9 zinc solderin a temperature range of 220° C. to 230° C.; and holding thetemperature of the reflow-soldered portion to be soldered at 178° C. orless to perform flow soldering.

When the temperature of the tin-9 zinc solder is controlled in a rangeof 220° C. to 230° C. to perform the flow soldering, the temperature ofthe reflow-soldered portion to be soldered is held at 178° C. or less asthe melting point of the ternary eutectic alloy of tin-silver-lead.

Therefore, the ternary eutectic alloy of tin-silver-lead formed in theinterface of the fillet and land in a micro size is not molten, and thefillet peel is solved

Moreover, since the solder alloy does not contain bismuth, the solder ofthe portion to be soldered is not embrittled.

(2) Further, in the above soldering method (1), when the printed wiringboard including the plated through hole is used, the printed wiringboard subjected to the HAL treatment by the tin-zinc solder is used.

When the printed wiring board subjected to the HAL treatment by thetin-zinc solder is used, during the flow soldering by the tin-9 zincsolder, the tin-9 zinc solder can easily wet up in the plated throughhole, and the tin-9 zinc solder wets/spreads not only over the land ofthe flow-soldered surface but also over the land of the reflow-solderedsurface. Therefore, the soldering superior in the soldering quality canbe performed.

(3) The flux for use in the flow soldering contains the acrylic acidadduct of rosin and the activator including the sarcosine framework.When the acrylic acid adduct of rosin and the activator including thesarcosine framework are used in the flux composition, the compositioncan easily wet up in the through hole during the flow soldering by thetin-9 zinc solder, and the soldering superior in the soldering qualitycan be performed.

Specifically, according to the present invention, the portions to besoldered exist in opposite surfaces of the printed wiring board, and thesoldering is performed on one surface side by the reflow solderingmethod and on the other surface side by the flow soldering method. Evenwhen the micro ternary eutectic alloy of the tin-silver-lead as a causefor the land peel is formed in the interface of the portion to besoldered by the reflow soldering, the lead-free solder not containingbismuth can be used to flow-solder the other surface side withoutexceeding the melting point of the ternary eutectic alloy oftin-silver-lead. Since this soldering method is established, the landpeel with the use of the lead-free solder is surely prevented, and thesoldering/mounting of the printed wiring board superior in solderingquality and reliability is possible.

(Third Aspect of Invention)

To achieve the third object, according to the present invention, thereis provided a flow soldering method for bringing a printed wiring boardon which an electronic component is mounted into contact with a jet flowof a solder containing tin and zinc as main components, that is, atin-zinc solder to perform soldering, the method comprising the stepsof: forming a layer of a solder rich in tin not containing bismuth andlead or a layer of tin on a land to be soldered and the surface of aplated through hole of the printed wiring board beforehand; thereaftermounting the electronic component; and subsequently bringing the printedwiring board into contact with the jet flow of the tin-zinc solder tosolder the board.

Specifically, the layer of the tin-rich solder or tin is formed on theland to be soldered and the surface of the plated through hole of theprinted wiring board, so that the wettability of the tin-zinc solder orwetting-up property in the plated through hole are enhanced, andsatisfactory soldering quality is obtained.

It is to be noted that the tin-rich solder containing bismuth is notused. This is because the solder after the soldering is embrittled andsuperior tenacity of the solder containing tin and zinc as the maincomponents is blocked. Of course, lead is not used for the lead-freesolder.

The tin-rich solder not containing bismuth and lead for use in theabove-described flow soldering method is any one of a solder containingtin, silver, and copper as the main components, that is, atin-silver-copper based solder, a solder containing tin and silver asthe main components, that is, a tin-silver based solder, and a soldercontaining tin and copper as the main components, that is, a tin-copperbased solder. The solder has a content of tin of 90 weight % or more.

Specifically, when the tin-silver-copper based solder, tin-silver basedsolder, or tin-copper based solder is used, and the content of tin is 90weight % or more, satisfactory wet-spread property of the tin-zincsolder in the land to be soldered is obtained. Moreover, thesatisfactory wetting-up property of the tin-zinc solder in the platedthrough hole is obtained.

Further, since the tin-silver-copper based solder, tin-silver basedsolder, and tin-copper based solder are also broadly used as thelead-free solder, the existing leveler apparatus can be used to easilyform the land on the land to be soldered and plated through hole of theprinted wiring board.

In the flow soldering method, a palladium or palladium alloy layer isformed on the surface of a soldering terminal of the electroniccomponent to be mounted on the printed wiring board.

In other words, the palladium or palladium alloy layer on the solderingterminal surface exhibits superior wettability to the tin-zinc solder.Moreover, the tin-rich solder layer or tin layer formed on the land tobe soldered and plated through hole of the printed wiring board is alsosuperior in the wet-spread or wet-up of the tin-zinc solder. Therefore,the printed wiring board having superior soldering quality is obtained.

In the flow soldering method, the electronic component to be mounted onthe printed wiring board includes the lead terminal, the palladium orpalladium alloy layer is formed on the lead terminal surface, and thelead terminal is inserted in the plated through hole and mounted.

In this flow soldering method, the tin-rich solder layer or tin layer isdisposed on a plated through hole inner wall, and the palladium orpalladium alloy layer is formed on the lead terminal surface of theelectronic component inserted in the plated through hole. Therefore, thetin-zinc solder also indicates superior wettability with respect to thelead terminal of the electronic component and the plated through holeinner wall. At the contact time with the jet flow of the tin-zincsolder, the tin-zinc solder quickly wets up in the plated through hole,and satisfactory soldering can be performed.

In the flow soldering method, the temperature of the jet flow of thetin-zinc solder is in a range of 220° C. to 250° C.

It has been necessary to perform the flow soldering operation at atemperature higher than the melting point by 40° C. or more in orderthat another lead-free solder or related-art tin-lead solder obtainssatisfactory wettability. However, the tin-zinc solder can obtain thesatisfactory wettability and wet-up property at a temperature higherthan the melting point by 20° C. or more. Therefore, even when thetemperature of the jet flow of the solder is low at about 220° C., thesatisfactory wettability can be obtained. Moreover, at 220° C. or more,there is hardly a difference of superiority of wettability bytemperature.

Therefore, at a temperature in a range of 220° C. to 250° C. lower thanthat of the related-art solder, the satisfactory soldering operation canbe obtained.

The flux for use in the flow soldering contains the acrylic acid adductof rosin and the activator including the sarcosine framework.

Thereby, the heat resistance of the flux is enhanced, and accordinglypersistent wettability can be enhanced. In the contact of the printedwiring board with the jet flow of the tin-zinc solder, the wet spread ofthe tin-zinc solder in the land to be soldered and the wet-up of thetin-zinc solder in the plated through hole are secured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a lead frame of a semiconductor component;

FIG. 2 is a sectional view showing that a lead terminal of asemiconductor component is inserted in a printed wiring board includinga plated through hole;

FIG. 3 is a sectional view showing a soldering state of a soldering leadterminal whose semiconductor drops;

FIGS. 4A and 4B are bird's-eye views of lift-off generated in alead-free solder;

FIG. 5 is a diagram of a soldering procedure showing the conventionalsoldering method of the present invention;

FIGS. 6A and 6B show an end surface of the portion to be solderedshowing the fillet peel, FIG. 6A is an explanatory view of a normalsoldering state, and FIG. 6B is an explanatory view of the state ofpeeled fillets 600 showing a peeled portion in an exaggerated manner forease of seeing;

FIG. 7 is a longitudinal sectional view showing an example of asoldering apparatus which realizes a soldering method of the presentinvention;

FIGS. 8A and 8B are diagrams showing a result of comparison of asoldering property in various solders and plating;

FIG. 9 is a diagram showing a relation between the lift-off in Sn—Pbplating and occurrence rate of land peel;

FIG. 10 is a diagram showing a relation between the lift-off in Auplating and occurrence rate of land peel;

FIG. 11 is a diagram showing a relation between the lift-off in Ni/Pdplating and occurrence rate of land peel;

FIG. 12 is a diagram showing a soldering procedure which realizes thesoldering method of the present invention;

FIG. 13 is a side sectional view, that is, longitudinal sectional viewshowing one example of a reflow soldering apparatus;

FIG. 14 is a diagram showing a constitution example of a flow solderingapparatus;

FIGS. 15A, 15B are tables showing a result of confirmation of occurrenceconditions of fillet peel, FIG. 15A shows types of a solder, types ofplating of an electronic component, and a result of analysis of asoldering procedure stage in which the peel occurs, FIG. 15B shows aresult of measurement of temperature of a portion to be solderedcompletely soldered by reflow soldering during the flow soldering;

FIG. 15C is a diagram showing a heat insulation tape for inhibiting heatconduction being attached to a surface to be flow-soldered opposite to asurface reflow-soldered with QFP IC before the flow soldering;

FIGS. 16A and 16B show tables showing the result of check ofpresence/absence of fillet peel using the soldering method of thepresent invention, FIG. 16A is a table collectively showing the types ofthe solder for the reflow soldering, types of plating of the electroniccomponent, and the temperature of the tin-9 zinc solder for use in theflow soldering as factors, and presence/absence of the fillet peel, andFIG. 16B is a table comparing the temperatures of portions during theflow soldering in the type of the solder and appropriate soldertemperatures for use in the soldering;

FIGS. 17A to 17E are explanatory views of a procedure for realizing themethod of flow soldering of the present invention;

FIG. 18 is a longitudinal sectional view showing one example of the flowsoldering apparatus;

FIG. 19 is an explanatory view of a spread solder height (H) and adiameter (D) of a volume of printed solder which is regarded as a ballin a spread ratio test of the solder in conformity to JIS-Z-3184;

FIGS. 20A and 20B show a diagram and a graph, respectively, collectivelyshowing spread ratio test results of the solder in conformity toJIS-Z-3184;

FIGS. 21A and 21B show a table and a diagram, respectively, showing theresults of the flow soldering by the tin-zinc solder of the printedwiring board subjected to various leveler treatments; and

FIGS. 22A and 22B show a table and a diagram, respectively, showingsoldering evaluation results using examples and comparative examples ofthe flux.

DESCRIPTION OF THE PREFERRED EMBODIMENT First Embodiment

A soldering method and solder joint member according to a firstembodiment of the present invention will be described hereinafter indetail with reference to the drawings.

First, a constitution of a soldering apparatus will be described withreference to FIG. 7. FIG. 7 is a longitudinal sectional view showing anexample of the soldering apparatus which realizes the soldering methodof the present invention. It is to be noted that a nitrogen gas supplysystem is shown by symbols.

Initially, lands and through holes of a printed wiring board are coatedwith a tin-zinc solder (tin-9 weight % zinc solder) by a HAL treatment.A method of performing the HAL treatment on the printed wiring board (bythe tin-zinc solder) is known and therefore omitted from the drawing.For example, the method of performing the HAL treatment is described inJapanese Patent Application Laid-Open No. 2001-168513.

Here, as described above, the HAL treatment is, for example, a treatmentfor spraying heated air to the substrate dipped in a bath of moltensolder to level the substrate in a method of coating a substrate withthe solder and soldering an electronic component onto the coating inorder to enhance a soldering property in the electronic circuitsubstrate.

Subsequently, the electronic component (e.g., semiconductor componenthaving a lead terminal type soldering terminal) including a solderinglead terminal plated with palladium or palladium alloy is inserted inthe through hole of the printed wiring board and mounted (see FIG. 2),and a work to be soldered is formed.

On the other hand, for the printed wiring board on which the electroniccomponent including the work to be soldered, that is, the soldering leadterminal plated with palladium is mounted, after flux is applied to asurface to be soldered by a flux application apparatus (not shown) asusual, the soldering is performed by a soldering apparatus 100 shown inFIG. 7.

The soldering apparatus 100 shown in FIG. 7 is constituted to perform asoldering operation in atmosphere of an inactive gas, namely, nitrogengas having low oxygen concentration.

Specifically, a conveyor for conveying the work to be soldered, that is,printed wiring board 101 is constituted of a first conveyor 102 forelevation-angle conveyance (elevation angle θ1) and second conveyor 103for depression-angle conveyance (depression angle θ2). A tunnel-shapedchamber 104 is disposed to cover these conveyors.

As shown in FIG. 7, a longitudinal section of the tunnel-shaped chamber104 has an inverted V shape, and a conveying-in port 105 andconveying-out port 106 are constituted to have the same height from ahorizontal plane. Since the height of the conveying-in port 105 isconstituted to be the same as that of the conveying-out port 106, it iseasy to connect the soldering apparatus 100 to another apparatus and usethe apparatus in line. Moreover, it is most important to easily retainatmosphere in the chamber at a temperature higher than that of theatmosphere outside the chamber 104, and the atmosphere in whichcomponents (e.g., oxygen concentration) are stable can be formed.

The first and second conveyors 102, 103 include holding claws forholding opposite side ends of the printed wiring board 101, and eachconveyor includes two conveyor frames disposed on opposite side ends andin parallel with each other. Further, one conveyor frame is usuallyconstituted to be movable/adjustable in a width direction of the printedwiring board 101, so that the printed wiring boards 101 having differentwidths can be held. A shown arrow A shows a conveyance direction of theprinted wiring board 101.

Moreover, a preheater 107 for preliminarily heating the printed wiringboard 101, and a solder tank 108 for performing a soldering step aredisposed along the first conveyor 102 in the tunnel-shaped chamber 104.

The preheater 107 of the preliminary heating step preliminarily isdisposed to heat the printed wiring board 101 to which flux has beenapplied beforehand and to reduce prepositive activation of the flux andheat shock given to the printed wiring board 101 and mounted electroniccomponent.

Further, the solder tank 108 of the soldering step contains a tin-zincsolder 109 (tin-9% zinc solder) which is heated by a heater (not shown)and which has a molten state. This molten solder 109 is sent to a firstspray nozzle 111 by a first pump 110 to form a first jet flow 112.Moreover, a second pump 113 sends the solder to a second spray nozzle114 to form a second jet flow 115.

Subsequently, these jet flows 112, 115 are brought into contact with thelower surface of the printed wiring board 101, that is, the surface tobe soldered on which a portion to be soldered 44 including a land 41,through hole 42, and soldering lead terminal 43 of the electroniccomponent exists as shown in FIG. 2. Thereby, the molten solder issupplied to the portion to be soldered 44 to solder the portion.

The preheater 107 is disposed in the tunnel-shaped chamber 104. However,the solder tank 108 is constituted such that an opening 116 is disposedin the tunnel-shaped chamber 104 and the first and second jet flows 112and 115 are positioned in the tunnel-shaped chamber 104 through theopening 116. With such a structure, a skirt 117 is disposed in theopening 116 formed in the tunnel-shaped chamber 104 to maintain thesealing of the tunnel-shaped chamber 104. The skirt 117 is immersed inthe molten solder in the solder tank 108 to realize complete sealing.

Further, a large number of plate members, namely, inhibit plates 118 aredisposed along the longitudinal direction of the tunnel, that is, theconveyance direction of the conveyor in the tunnel-shaped chamber 104.Additionally, the inhibit plates 118 are disposed so that plate surfacescross at right angles to the conveyance direction of the conveyor.Specifically, the inhibit plates 118 form a labyrinth seal in thetunnel-shaped chamber 104, and are constituted to prevent an unnecessaryatmospheric flow from being generated in the tunnel-shaped chamber 104.

In this event, the inhibit plates 118 are disposed down to the conveyorfrom an upper wall of the tunnel-shaped chamber 104 and disposed up tothe conveyor from a lower wall of the tunnel-shaped chamber 104.

Discharge ports 119 for supplying nitrogen gas as inactive gas into thetunnel-shaped chamber 104 are disposed among the inhibit plates 118disposed after the solder tank 108 as viewed from the conveyancedirection, and a target nitrogen gas supply flow rate can be adjusted byflow rate adjustment valves 120 and flow rate meters 121. The nitrogengas is supplied from a nitrogen gas supply apparatus 122 of a bomb orPSA type, and supplied to the flow rate adjustment valves 120 via anopen/close valve 123, filter 124 for removing impurities, and pressurecontrol valve 125 for adjusting a supply pressure to a target. Apressure meter 126 is disposed to monitor the pressure.

For the nitrogen gas supply flow rate, oxygen concentration in thetunnel-shaped chamber 104 is measured by an oxygen concentration meter(not shown), for example, an atmosphere of the soldering step as acontact region of the printed wiring board 101 with the jet flow of themolten solder 109 is sampled, and the rate of the sampled region ismeasured. The flow rate adjustment valves 120 are adjusted so as toobtain the target oxygen concentration, and the flow rate is set.

Moreover, if necessary, for a constitution as shown by a broken line,the discharge port 119 for nitrogen gas supply is similarly disposed inthe vicinity of the preheater 107 of the preliminary heating step, andthe oxygen concentration of the atmosphere in the vicinity of thepreheater 107 may also be measured by the oxygen concentration meter.

Next, an operation of the soldering apparatus 100 shown in FIG. 7 willbe described below.

The printed wiring board 101 in which the flux has been appliedbeforehand to the lower surface of the printed wiring board 101including the portion to be soldered, that is, the surface to besoldered is conveyed through the conveying-in port 105 of the solderingapparatus 100 shown in FIG. 7. Then, the opposite side ends of the boardare held by the holding claws of the first conveyor-102, and conveyed inthe arrow A direction at the conveyance elevation angle θ1.

Subsequently, for example, the portion to be soldered is preliminarilyheated at about 100° C. by the preheater 107. Successively, the lowersurface of the printed wiring board 101, namely, the surface to besoldered is brought in contact with the first and second jet flows 112and 115, for example, at a temperature of about 250° C., and the moltensolder 109 is supplied to the portion to be soldered to solder theportion.

Thereafter, the printed wiring board 101 is transferred to the secondconveyor 103 at the vertex of the tunnel-shaped chamber 104, conveyed atthe conveyance depression angle θ2, and conveyed out of theconveying-out port 106 to complete the soldering.

This series of soldering operation is performed in the nitrogen gasatmosphere having low oxygen concentration. Specifically, the inside ofthe tunnel-shaped chamber 104 turns to the nitrogen gas atmospherehaving the low oxygen concentration by the nitrogen gas supplied fromthe discharge port 119 for the nitrogen gas supply.

By the series of soldering operation constituted as described above, themolten solder from the jet flow of the solder containing tin and zinc asthe main components is supplied to the soldering land and plated throughhole on which the solder layer containing tin and zinc as maincomponents is formed. The solder is similarly supplied to the solderinglead terminal of the electronic component which is inserted in thethrough hole and which includes a palladium or palladium alloy layer onthe surface of the terminal. Thereby, the tin-zinc solder wets thesoldering lead terminal and wets up in the plated through hole, and thesoldering also wet-spreads over the land disposed on a componentinsertion surface (the upper surface of the printed wiring board shownin FIGS. 2 and 3) to form a fillet (see 55 of FIG. 3). A satisfactorysolder joint member superior in wettability can be obtained.

In other words, palladium and palladium alloy exhibit satisfactorywettability with respect to the solder containing tin and zinc as themain components. On the soldering land formed of copper and the surfaceof the plated through hole, the tin-zinc solder layer in which thesolder containing tin and zinc as the main components is sufficientlywet is formed by the HAL treatment.

First Embodiment

FIGS. 8A and 8B show a table and a diagram, respectively, showing aresult of comparison of a soldering property, and FIG. BA is a tablecollectively showing comparison results. FIG. 8B is a longitudinalsectional view of the portion to be soldered showing an evaluationstandard (level) indicating a degree at which the molten solder wets upand is filled in the through hole of the portion to be soldered.

First, to evaluate the soldering property, the evaluation standard wasdetermined. Specifically, as shown in FIG. 8B, “O” indicates asatisfactory soldered state in which the solder wets up in a throughhole 22 and solder fillets are formed over lands 21 on lower and uppersurfaces of a printed wiring board 20. A solder fillet 24 is formed onthe land 21 of the lower surface of the printed wiring board 20 and thesolder wets up in the through hole 22, However, the solder cannotwet-spread over the upper land 21 of the printed wiring board 20 and thefillet 24 is not formed. This state is indicated as a slightlyinsufficient soldered state by “Δ.” The solder fillet 24 is formed onthe land 21 of the lower surface of the printed wiring board 20, but thesolder does not sufficiently wet up in the through hole 22. Therefore,the solder cannot wet-spread over the upper land 21 of the printedwiring board 20 and the fillet 24 is not formed. This state is shown asan insufficient soldered state by “x.”

On the other hand, to compare the soldering property, as theconstitution of the portion to be soldered, three types having surfacetreated states of the soldering lead terminal were prepared.Specifically, the soldering lead terminal plated with a tin-10 weight %lead solder containing conventional lead, soldering lead terminal platedwith gold, and soldering lead terminal including a nickel layer as anunderlayer and a palladium or palladium alloy layer formed on the nickellayer were prepared.

In the meantime, as surface treated states of the land and through holeof the printed wiring board as another constituting element of theportion to be soldered, four types were prepared. Specifically, thefollowings were prepared: a. printed wiring board on which copper usedas a wiring pattern or land is exposed; b. printed wiring boardincluding the copper land and through hole subjected to the HALtreatment with a tin-37 weight % lead solder; c. printed wiring boardincluding the copper land and through hole HAL-treated with a tin-9weight % zinc solder; and d. printed wiring board obtained byHAL-treating the surface of the printed wiring board with the tin-9weight % zinc solder and further applying antirust agent to the surface.

Moreover, as the types of the solder to be supplied to the portion to besoldered from the jet flow in the soldering step, the followings wereprepared: a. a tin-37 weight % lead solder heretofore used; b. a tin-3weight % silver-0.5 weight % copper solder as a potential candidate of alead-free solder; c. tin-0.7 weight % copper solder; and d. tin-9 weight% zinc solder. It is to be noted that the temperature of the solder wasset to a usually used solder temperature of 250° C.

Further, a relation of soldering property among these soldering leadterminals, printed wiring boards a to d, and solders a to d iscollectively shown in a table of FIG. 8A.

Specifically, for the soldering lead terminals plated with the tin-37%zinc solder and gold, satisfactory soldering properties were obtainedregardless of the types of the surface treatment of the land and throughhole and the solder. However, from a demand for a lead-free constitutionoriginated in an environmental problem, the plating treatment of thetin-10 weight % lead solder cannot be continued as the surface treatmentof the soldering lead terminal of the electronic component in thefuture. Moreover, gold is extremely expensive, is therefore used onlyfor a special purpose in which cost does not raise a problem, and is notused in mostly used general-purpose electronic component.

On the other hand, with the soldering lead terminal plated withpalladium, the printed wiring board whose land and through hole areHAL-treated by the tin-9 weight % zinc solder is used, and the solder issupplied from the jet flow of the tin-9 weight % zinc solder. It is seenthat the satisfactory soldering property is obtained only in this case.

That is, considering from mounting reliability of the electroniccomponent manufactured by these all mounting operations from a viewpointof a series of all mounting steps of manufacturing the semiconductorcomponent and soldering/mounting the component onto the printed wiringboard, for a palladium-plated lead frame, die bonding property by silverwax and wire bonding property by a gold wire are superior. During thesoldering/mounting, the component is mounted on the printed wiring boardHAL-treated by the tin-zinc solder, and the molten solder is suppliedfrom the jet flow of the tin-zinc solder to solder the component. It isfound out that most superior mounted member can be formed in this case.

Subsequently, to compare lift-off occurrence rates, three types wereprepared as the plating treatment of an electronic component solderinglead terminal portion. Specifically, the followings were prepared: (1) asoldering lead terminal plated with the tin-10 weight % lead soldercontaining conventional lead; (2) a soldering lead terminal includingthe nickel layer as the underlayer and gold plating on the upper layerof the nickel layer; and (3) a soldering lead terminal including thenickel layer as the foundation and the palladium or palladium alloylayer formed on the nickel layer. On the other hand, as the surfacetreated state of the through hole of the printed wiring board as theother constituting element of the portion to be soldered, two types wereprepared.

Specifically, the followings were prepared: a) a printed wiring board inwhich pre-flux is applied as an antioxidant on copper used as the wiringpattern; and b) a printed wiring board whose through hole is HAL-treatedwith the tin-9 weight % zinc solder and which is 1.6 mm thick. Further,for through hole and land designs of the printed wiring board, ninetypes were prepared.

That is, the followings were prepared: 1) a through hole diameter of 0.8mm, land diameter of 1.1 mm; 2) through hole diameter of 0.8 mm, landdiameter of 1.4 mm; 3) through hole diameter of 0.8 mm, land diameter of1.7 mm; 4) through hole diameter of 1.0 mm, land diameter of 1.3 mm; 5)through hole diameter of 1.0 mm, land diameter of 1.6 mm; 6) throughhole diameter of 1.0 mm, land diameter of 1.9 mm; 7) through holediameter of 1.2 mm, land diameter of 1.5 mm; 8) through hole diameter of1.2 mm, land diameter of 1.8 mm; and 9) through hole diameter of 1.2 mm,land diameter of 2.1 mm.

Moreover, as the types of the solder to be supplied to the portion to besoldered from the jet flow in the soldering step, the followings wereprepared: a) a tin-3 weight % silver-0.5 weight % copper solder as thepotential candidate of the lead-free solder; and b) tin-9 weight % zincsolder. It is to be noted that the temperature of the solder was set tothe usually used solder temperature of 250° C.

A relation of the lift-off occurrence rates among these soldering leadterminals 1 to 3, printed wiring boards a, b, and solders a, b iscollectively shown in tables of FIGS. 9 to 11. It is to be noted that inthese diagrams T/H diameter denotes the through hole diameter and Landdiameter denotes the land diameter.

Specifically, as shown in FIG. 9, in (1) the soldering lead terminalplated with the tin-10 weight % lead solder including conventional lead,for the printed wiring board to which pre-flux was applied and the tin-3weight % silver-0.5 weight % copper solder as the potential candidate ofthe lead-free solder, the lift-off occurrence rate was 50%. The printedwiring board whose through hole was HAL-treated with the tin-9 weight %zinc solder had a lift-off occurrence rate of 50% or less. Reductioneffect was recognized.

In the meantime, as shown in FIG. 11, in the soldering lead terminalincluding the nickel layer as the underlayer and the palladium orpalladium alloy layer formed in the upper layer of the nickel layer, thetin-3 weight % silver-0.5 weight % copper solder as the potentialcandidate of the lead-free solder and the tin-9 weight % zinc solder hadan equal lift-off occurrence rate of 0. Moreover, as shown in FIG. 10,for the soldering lead terminal including the nickel layer as thefoundation and the gold-plated upper surface of the nickel layer, insome combination of the through hole and land diameters, the occurrenceof lift-off was recognized in the tin-3 weight % silver-0.5 weight %copper solder. However, the lift off occurrence rate was zero with thetin-9 weight % zinc solder. It has been seen that the occurrence iscompletely prevented.

Meanwhile, as compared with the conventional tin-lead solder, thetin-zinc based solder as the lead-free solder has a high melting pointand bad wettability, because tin and zinc are the main components.Therefore, when the conventional post flux is used, sufficientwettability and joint reliability cannot easily be secured.

Therefore, in the present invention, from a viewpoint of enhancement ofheat resistance of post flux and enhancement of solder wettability,instead of gum rosin as a resin component of the conventional post flux,the acrylic acid adduct of rosin having higher heat resistance wasselectively used.

According to the present invention, from a viewpoint that persistentwettability as required capability of the post flux for use in thetin-zinc based solder should be secured, the activator to be blended wasnoted, and thereby the activator including the sarcosine framework wasselectively used so that the persistent wettability was possible. Theacrylic acid adduct of rosin for use in the present invention includesan addition reactant of acrylic acid and rosin or a rosin derivativefurther subjected to hydrogenation reaction. Examples of rosin includegum rosin, wood rosin, and tall oil rosin containing resin acids such asabietic acid, palustric acid, neoabietic acid, pimaric acid, isopimaricacid, and dehydroabietic acid as main components.

Examples of the activator including the sarcosine framework for use inthe present invention include oleyl sarcosine, lauroyl sarcosine, andpalm oil fatty acid sarcosine. Furthermore, in addition to theabove-described specific constituting components, with the flux of thepresent invention, various components for use in a conventional flux canalso be used such as resin, activator, solvent, antioxidant,delusterant, and other additives. Examples of the resin component foruse in the conventional flux include various rosin derivatives such asgum rosin, polymerized rosin, deproportionated rosin, and hydrogenatedrosin, and synthetic resins such as a polyamide resin and terpene resin.

Examples of the activator for use in the conventional flux includehydrohalide of amine, organic acids, organic amine, and organic halide.Concrete examples of hydrohalide of amine include diethylaminehydrochloride, and cyclohexylamine hydrobromide. Concrete examples oforganic acids include adipic acid, stearic acid, and benzoic acid.Concrete examples of organic amines include hexylamine, dibutylamine,and triethylamine. Concrete examples of organic halides include2,3-dibromo-1-propanol, and 2,3-dibromo-2-butene-1,4-diol. Examples ofthe solvent include isopropyl alcohol, acetic ether, and toluene.Furthermore, antioxidant, delusterant, and other additives are notespecially limited, and various known agents may appropriately beselected and used.

The present invention will be described hereinafter in detail in termsof examples. A liquid flux composition including Examples 1 to 4 andComparative Examples 1 to 3 was prepared, and spray-applied to theprinted wiring board on which the electronic component was mounted, andthe component was soldered by an automatic soldering apparatus. Asoldered state was observed, and the soldering property was evaluatedwith “O,” “Δ,” and “x.” The evaluation result is shown in Table 1 below.TABLE 1 (Example 1) Adipic acid 1.0 wt % Dibutyl amine 1.0 wt %Cyclohexylamine hydrobromide 0.2 wt % Oleyl sarcosine 1.0 wt % Acrylicacid adduct of rosin 7.0 wt % Hydrogenerated rosin 7.0 wt % Isopropylalcohol 82.8 wt % (Example 2) Sebacic acid 1.0 wt %2,3-dibromo-1-propanol 0.5 wt % Diethylamine hydrochloride 0.2 wt %Lauroyl sarcosine 1.0 wt % Acrylic acid adduct of rosin 10.0 wt % Gumrosin 5.0 wt % Isopropyl alcohol 82.3 wt % (Example 3) Octyl acid 1.0 wt% Benzoic acid 1.0 wt % Diphenylguanidine hydrobromide 0.2 wt % Palm oilfatty acid sarcosine 1.0 wt % Acrylic acid adduct of rosin 20.0 wt %Isopropyl alcohol 77.8 wt % (Example 4) Glutaric acid 1.0 wt %2,3-dibromo-1-propanol 0.5 wt % Cyclohexylamine hydrobromide 0.2 wt %Oleyl sarcosine 1.0 Wt % Acrylic acid adduct of rosin 7.0 wt %Deproportionated rosin 7.0 wt % Isopropyl alcohol 83.3 wt % (ComparativeExample 1) Adipic acid 1.0 wt % Dibutyl amine 1.0 wt % Cyclohexylaminehydrobromide 0.2 wt % Gum rosin 15.0 wt % Isopropyl alcohol 82.8 wt %(Comparative Example 2) Sebacic acid 1.0 wt % 2,3-dibromo-1-propanol 0.5wt % Diethylamine hydrochloride 0.2 wt % Lauroyl sarcosine 1.0 wt % Gumrosin 15.0 wt % Isopropyl alcohol 82.3 wt % (Comparative Example 3)Octyl acid 1.0 wt % Benzoic acid 1.0 wt % Diphenylguanidine hydrobromide0.2 wt % Acrylic acid adduct of rosin 10.0 wt % Hydrogenerated rosin10.0 wt % Isopropyl alcohol 77.8 wt %

TABLE 1 Ex. Ex. Ex. Ex. Compar. Compar. Compar. 1 2 3 4 Ex. 1 Ex. 2 Ex.3 Evaluation ◯ ◯ ◯ ◯ x Δ Δ Result

In other words, as shown in Table 1, satisfactory soldering propertiesare obtained in Examples 1, 2, 3. As described above, it is seen thatsatisfactory electric connection and mechanical connection havingsufficient strength as basic soldering joint conditions are obtained inthe soldering method and solder joint member of the present invention.Additionally, the tin-zinc solder has appropriate hardness and superiorelongation characteristic. For example, the related-art tin-37 weight %lead solder has a high-temperature elongation of 66% at 125° C., and thetin-9 weight % zinc solder has an elongation of 67% which is an equal ormore value. On the other hand, the tin-3 weight % silver-0.5 weight %copper solder has an elongation of only 12%.

Therefore, to solder/mount the semiconductor component including thelead frame with the palladium or palladium alloy layer formed on thesurface of the frame onto the printed wiring board, it is possible toobtain the satisfactory soldering property by the soldering method ofthe present invention. It is found out that it is possible to obtain thesolder joint member which is tough and is superior in mechanical andelectric connections and long-term reliability.

Second Embodiment

A second embodiment of the present invention will be describedhereinafter in detail with reference to the drawings.

(1) Soldering Procedure

A soldering procedure for realizing the soldering method of the presentinvention will be described with reference to FIG. 12. The order ofsteps (1) to (9) is substantially the same as that of FIG. 5, buttreatment content in each step is different from that of FIG. 5.

First in step (1), a printed wiring board 10 HAL-treated by the tin-zincsolder is prepared, and an adhesive 13 for bonding an electroniccomponent (SMD) 12 to be flow-soldered is applied. Subsequently in step(2), the corresponding electronic component 12 is mounted. Thereafter instep (3), the adhesive 13 applied in the step (1) is cured, and theelectronic component 12 is fixed onto lands 14.

Here, as described above, the HAL treatment is, for example, a treatmentfor spraying the heated gas (air) to the substrate dipped in the bath ofmolten solder to level the substrate in the method of coating thesubstrate with the solder and soldering the electronic component ontothe coating in order to enhance the soldering property in the electroniccircuit substrate.

Next in step (4) the printed wiring board 10 is reversed, and the creamsolder 15 of tin-3 silver-0.5 copper (solid phase line temperature of217° C., liquid phase line temperature of 220° C.) or cream solder 15 oftin-3.5 silver-0.75 copper (solid phase line temperature of 217° C.,liquid phase line temperature of 219° C.) are supplied beforehand ontothe lands 14 on which the soldering terminals of the electroniccomponent 12 to be reflow-soldered are laid. Successively, in step (5),the chip component 62 whose soldering terminal is coated, for example,plated with the tin-zinc solder and electronic components 16 such as QFPIC are mounted on the lands 14. Naturally, the electronic componentincluding another type of plating may also be included.

Thereafter, in step (6), the reflow soldering is performed by the reflowsoldering apparatus. Specifically, the solder supplied in theabove-described step is heated and molten, and the portions to besoldered of the chip component 62 and electronic components 16 such asthe QFP IC formed between soldering terminals 17 (lead terminalportions) and lands 14 are soldered.

After the reflow soldering is completed, in step (7), for example, leadterminals 19 of an insertion component 18 such as the connector isinserted in a through hole 11 on the side of the reflow soldered surfaceof the printed wiring board 10. Moreover, in step (8) the jet flow ofthe molten tin-9 zinc solder whose temperature is controlled in a rangeof 220° C. to 230° C. by a temperature control apparatus in step (8) isbrought into contact with the surface to be flow-soldered of the printedwiring board 10 to flow-solder the surface.

In other words, the portion to be soldered formed between the land 14and the electronic component 12 fixed on the land 14 in the step (3),and the portion to be soldered formed between the land 14 including theplated through hole 11 and the lead terminals 19 of the insertioncomponent 18 inserted in the through hole 11 in the step (7) are broughtinto contact with the jet flow of the molten tin-9 zinc solder. Thetin-9 zinc solder is supplied to these portions to be soldered toflow-solder the portions.

Since the portions are flow-soldered by tin-9 zinc in this flowsoldering step (8), the temperature is low at 220° C. to 230° C.Specifically, the portions can be flow-soldered at a very lowtemperature as compared with temperature of 245° C. to 250° C. regardedas appropriate in the related-art tin-37 zinc solder, or temperature of250° C. to 255° C. regarded as appropriate in the lead-free solder 15 oftin-3 silver 0.5 copper or tin-3.5 silver-0.75 copper. Moreover, sincethe printed wiring board 10 is HAL-treated by the tin-zinc solder, thewetting-up of the tin-9 zinc solder in the plated through hole 11 isalso extremely satisfactory.

In the flow soldering step (8), the solder of the reflow-solderedportion to be soldered, for details, the micro ternary eutectic alloy oftin-silver-lead formed between the lands 14 on which the components 16,62 are mounted and fillets 150 is not heated at temperature exceeding amelting point of 178° C., and fillet peel does not occur.

As a result, as shown in step (9), the portions to be soldered existingon the opposite surfaces of the printed wiring board 10 are cooled andcompletely soldered. The fillets 150 of the solder including no filletpeel and having a satisfactory joined state are formed on the portionsto be soldered of the SMD and insertion component 18, and the printedwiring board 10 superior in soldering quality is completed.

Moreover, since the flow soldering is performed by the tin-9 zincsolder, the lift-off phenomenon generated during the flow soldering canbe prevented or reduced more than another lead-free solder. Further,since the lead-free solder not containing bismuth is used, the solder ofthe portion to be soldered is not embrittled. It is possible to performthe soldering/mounting which is tough against heat cycle stress, stressby repeatedly applied acceleration, and stress of vibration.

Since the acrylic acid adduct of rosin and the activator including thesarcosine framework is used in the flux composition to be applied duringthe flow soldering, the wetting-up of the tin-9 zinc solder in thethrough hole 11 can further be improved.

In the present invention, from a viewpoint of enhancement of heatresistance of the post flux, instead of gum rosin as the resin componentof the related-art post flux, the acrylic acid adduct of rosin havinghigher heat resistance is selectively used.

Further, according to the present invention, from a viewpoint that thepersistent wettability as the required capability of the post flux foruse in the tin-zinc solder should be secured, the activator to beblended is also noted. The activator using the sarcosine framework isselectively used, so that the persistent wettability is possible.

The acrylic acid adduct of rosin for use in the present inventionincludes the addition reactant of acrylic acid and rosin or the rosinderivative further subjected to hydrogenation reaction. Examples ofrosin include gum rosin, wood rosin, and tall oil rosin containing resinacids as main components such as abietic acid, palustric acid,neoabietic acid, pimaric acid, isopimaric acid, and dehydroabietic acid.Moreover, examples of the activator including the sarcosine frameworkfor use in the present invention include oleyl sarcosine, lauroylsarcosine, and palm oil fatty acid sarcosine.

In addition to the above-described specific constituting components,with the flux of the present invention, various components for use inthe conventional flux can also be used such as resin, activator,solvent, antioxidant, delusterant, and other additives.

Examples of the resin component for use in the conventional flux includevarious rosin derivatives such as gum rosin, polymerized rosin,deproportionated rosin, and hydrogenated rosin, and synthetic resinssuch as a polyamide resin and terpene resin. Examples of the activatorfor use in the conventional flux include hydrohalide of amine, organicacids, organic amine, and organic halide. Concrete examples ofhydrohalide of amine include diethylamine hydrochloride, andcyclohexylamine hydrobromide. Concrete examples of organic acids includeadipic acid, stearic acid, and benzoic acid. Concrete examples oforganic amines include hexylamine, dibutylamine, and triethylamine.Concrete examples of organic halides include 2,3-dibromo-1-propanol, and2,3-dibromo-2-butene-1,4-diol. Examples of the solvent include isopropylalcohol, acetic ether, and toluene.

Further, antioxidant, delusterant, and other additives are notespecially limited, and various known agents may appropriately beselected and used.

(2) Constitution of Soldering Apparatus

Next, a constitution example and operation of a reflow solderingapparatus will be described with reference to FIG. 13.

The reflow soldering apparatus heats the portion to be soldered to whichthe solder is supplied beforehand to melt the solder, and solders theland and circuit pattern formed on the printed wiring board to theelectronic component.

Examples of a heating method of the reflow soldering apparatus includehot wire heating by infrared rays, hot air heating, steam heating, heattransfer heating, and combined heating. In any method, the printedwiring board is heated and reflow-soldered in a chamber type heatingfurnace in order to maintain high-temperature atmosphere. Moreover, theboard is often heated in atmosphere having low oxygen concentration inwhich inactive gases such as nitrogen gas are supplied.

FIG. 13 is a side sectional view, that is, longitudinal sectional viewshowing one example of a reflow soldering apparatus 200. It is to benoted that the nitrogen gas supply system is shown in a symbol diagram.

In this example, the reflow soldering apparatus has a so-calledseven-zone constitution in which a heating furnace, that is, furnacemember 216 is divided in seven regions. A heating chamber 204 includingheating means 203 is vertically symmetrically disposed in which heatingmeans 203 are disposed via a conveyor 202 for conveying a printed wiringboard 201, which is a boundary, and 14 heating chambers 204 aredisposed. Moreover, first and second zones correspond to a temperaturerise portion heating chamber along a conveyance direction A of theprinted wiring board 201, third to fifth zones correspond to a holdingheating chamber, and sixth and seventh zones correspond to a reflowportion heating chamber.

Known hot air heating means and infrared heating means which are heatingmeans 203 are used. Moreover, these heating means 203 are combined andused. For a supply heat amount to the printed wiring board 201, in thehot air heating means a final supply heat amount is determined by a hotair temperature, and a supply heat amount per unit time is determined bya hot air velocity.

Further, in the infrared heating means, the supply heat amount can beincreased in proportion to substantially the fourth power of the surfacetemperature of an infrared heater. Additionally, the hot airtemperature, hot air velocity, and surface temperature of the infraredheater are adjusted to thereby adjust the heating temperature of theprinted wiring board 201, so that a targeted heating temperature profileis obtained.

Details of the conveyor 202 are not shown, and the conveyor is usuallyused to hold and convey the printed wiring board 201 by opposite sideends by two parallel conveyance chains. Herein, labyrinth portions,namely, inlet labyrinth portion 207 and outlet labyrinth portion 208 areconstituted by inhibit plates disposed in parallel in a conveying-inport 205 and conveying-out port 206. Sealing properties inside andoutside the heating chamber 204 and outside the furnace member 216 aresecured.

In the meantime, nitrogen gas is supplied to a reflow portion heatingchamber in which the solder is molten, and nitrogen gas atmospherehaving lowest oxygen concentration is formed in a reflow portion heatingchamber. Specifically, after unnecessary materials are removed by afilter 211 through an open/close valve 210 from a nitrogen gas supplyapparatus 209 of bomb or PSA type, a target pressure is maintained by apressure control valve 212. A target flow rate is adjusted by a flowrate control valve 213 and flow rate meter 214, and the gas is supplied.A pressure meter 215 is for the pressure monitor.

The soldering operation is performed while the printed wiring board 201is conveyed by the conveyor 202. Specifically, the printed wiring board201 supplied through the conveying-in port 205 is conveyed in the arrowA direction. The board is passed and heated through the respectiveheating chambers 204 in order of the temperature rise portion→holdingportion→reflow portion. The completely soldered printed wiring board 201is conveyed out via the conveying-out port 206.

A heating reach temperature of the printed wiring board 201 in eachheating chamber 204 is adjusted by the heat amount supplied to theprinted wiring board 201 from the heating means 203 of each heatingchamber 204, and is further adjusted by the hot air temperature, hot airvelocity, and surface temperature of the infrared heater. Thereby, theprinted wiring board 201 is heated and reflow-soldered with the targetedpredetermined heating temperature profile.

For example, with the tin-silver-copper based lead-free solder for usein the present embodiment, that is, the cream solder of tin-3 silver-0.5copper or tin-3.5 silver-0.75 copper, it is usual to control thetemperature of the portion to be soldered in the reflow portion in atemperature range of about 230° C. to 240° C. and perform the soldering.

It is to be noted that the reflow soldering can also be performed in theatmospheric air without supplying nitrogen gas into the heating chamber204 during the reflow soldering operation of the tin-silver-copper basedlead-free solder. Additionally, when the reflow soldering is performedin the nitrogen gas atmosphere having low oxygen concentration, theportion to be soldered or solder of the printed wiring board 201 isprevented from being oxidized. The soldering can be performed with moresatisfactory wettability than in the atmospheric air.

Next, the constitution example and operation of the flow solderingapparatus will be described with reference to FIG. 14. Here, FIG. 14 isa longitudinal sectional view showing one example of the flow solderingapparatus, and the nitrogen gas supply apparatus is shown in the symboldiagram.

A flow soldering apparatus 300 is constituted to perform the solderingoperation in the atmosphere of the inactive gas, that is, nitrogen gashaving the low oxygen concentration. It is to be noted that the flowsoldering apparatus of FIG. 14 has a constitution similar to that of theflow soldering apparatus of FIG. 7.

Specifically, the conveyor for conveying the work to be soldered, thatis, printed wiring board 301 is constituted of a first conveyor 302 forelevation-angle conveyance (elevation angle θ1) and second conveyor 303for depression-angle conveyance (depression angle θ2). A tunnel-shapedchamber 304 is disposed to cover these conveyors.

As shown in FIG. 14, a longitudinal section of the tunnel-shaped chamber304 has an inverted V shape, and a conveying-in port 305 andconveying-out port 306 are constituted to have the same height from thehorizontal plane. Thereby, it is easy to retain the atmosphere in thechamber 304 at the temperature higher than that of the atmosphereoutside the chamber 304, and the atmosphere in which the components(e.g., oxygen concentration) are stable can be formed.

The first and second conveyors 302, 303 include holding claws forholding the opposite side ends of the printed wiring board 301, and eachconveyor includes two conveyor frames disposed on opposite side ends andin parallel with each other. Moreover, a preheater 307 constituting thepreliminary heating step of the printed wiring board 301, and a soldertank 308 constituting the soldering step are disposed along the firstconveyor 302 in the tunnel-shaped chamber 304. It is to be noted thatthe shown arrow A indicates the conveyance direction of the printedwiring board 301.

The solder tank 308 of the soldering step contains a tin-9 zinc solder309, and the solder 309 is heated by the heater, temperature sensor, andtemperature control apparatus (not shown) and which has a molten state.The temperature is controlled to a temperature indicated beforehand.Moreover, this molten solder 309 is sent to a first spray nozzle 311 bya first pump 310 to form a first jet flow 312. Furthermore, a secondpump 313 sends the solder to a second spray nozzle 314 to form a secondjet flow 315.

Subsequently, these jet flows 312, 315 are brought into contact with thelower surface of the printed wiring board 301, namely, the surface to beflow-soldered of the lower surface of the printed wiring board 10 onwhich the portion to be soldered exists including the land 14 andthrough hole 11, and the soldering and lead terminals of the electroniccomponent 12 as shown in the step (7) of FIG. 12. Thereby, the moltensolder 309 is supplied to the portion to be soldered to solder theportion.

Moreover, the preheater 307 is disposed in the tunnel-shaped chamber304. However, the solder tank 308 is constituted such that an opening316 is disposed in the tunnel-shaped chamber 304 and the first andsecond jet flows 312 and 315 are positioned in the tunnel-shaped chamber304 through the opening 316. It is to be noted that a skirt 317 isdisposed in the opening 316. The skirt 317 is immersed in the moltensolder 309 in the solder tank 308 to realize the complete sealing.

Furthermore, a large number of plate members, that is, inhibit plates318 are disposed along the longitudinal direction of the tunnel, thatis, the conveyance direction of the conveyors 302, 303 in thetunnel-shaped chamber 304. The plates form the labyrinth seal, and areconstituted to prevent an unnecessary atmospheric flow from beinggenerated in the tunnel-shaped chamber 304.

Discharge ports 319 for supplying nitrogen gas which is the inactive gasinto the tunnel-shaped chamber 304 are disposed among the inhibit plates318 disposed after the solder tank 308 as viewed from the conveyancedirection, and the targeted nitrogen gas supply flow rate can beadjusted by flow rate adjustment valves 320 and flow rate meters 321.The nitrogen gas is supplied from a nitrogen gas supply apparatus 322 ofa bomb or PSA type, and supplied to the flow rate adjustment valves 320via an open/close valve 323, filter 324 for removing impurities, andpressure control valve 32S for adjusting a supply pressure to thetarget. A pressure meter 326 is disposed to monitor the pressure.

For the nitrogen gas supply flow rate, the oxygen concentration in thetunnel-shaped chamber 304 is measured by the oxygen concentration meter(not shown), for example, the atmosphere of the soldering step as thecontact region of the printed wiring board 301 with the jet flows 312,315 of the molten solder 309 is sampled, and the rate is measured. Theflow rate adjustment valves 320 are adjusted so as to obtain the targetoxygen concentration, and the flow rate is set.

Moreover, if necessary, for the constitution as shown by a broken line,the discharge port 319 for nitrogen gas supply is similarly disposed inthe vicinity of the preheater 307 of the preliminary heating step, andthe oxygen concentration of the atmosphere in the vicinity of thepreheater 307 may also be measured by the oxygen concentration meter. Inthis constitution, the oxygen concentrations of the preliminary heatingstep and soldering step can also separately be adjusted/controlled.

The soldering operation is performed while the printed wiring board 301is conveyed by the conveyors 302, 303. Specifically, the printed wiringboard 301 supplied through the conveying-in port 305 is conveyed in thearrow A direction. The board is conveyed in order of the preliminaryheating step→soldering step, the molten solder 309 is supplied to theportion to be soldered of the printed wiring board 301 by the jet flows312, 315 in the soldering step to solder the portion, and the completelysoldered printed wiring board 301 is conveyed out via the conveying-outport 306.

The flow soldering operation is performed in the preliminary heatingstep, while the temperature of the portion to be soldered of the printedwiring board 301 indicates a predetermined target value in a range of80° C. to 150° C. The operation is performed in the soldering step,while the temperature of the tin-9 zinc solder indicates thepredetermined target value in a range of 220° C. to 230° C.

It is to be noted that, as disclosed in Japanese Patent ApplicationLaid-Open No. 2001-293559, the oxygen concentration of the preliminaryheating step is controlled at 1000 ppm or less, the oxygen concentrationof the soldering step is controlled at 500 ppm or less, and the printedwiring board 301 by the tin-9 zinc solder can be flow-soldered withsatisfactory wettability.

Second Embodiment

As clarified by the present inventors, when the soldering terminal ofthe electronic component is coated with the tin-lead solder, and whenthe reflow soldering is performed with the tin-silver-copper basedlead-free solder, a micro ternary eutectic alloy of tin-silver-leadhaving a melting point of 178° C. is formed in a fillet interface of theland of the portion to be soldered. This causes the fillet peel in thesubsequently performed flow soldering.

Then, the present inventors further confirmed this fact in the actualsoldering operation of the printed wiring board.

FIGS. 15A through 15C show tables and a diagram showing a result ofconfirmation of occurrence conditions of the fillet peel, and FIG. 15Ashows types of the solder, types of plating of the electronic component,and the result of analysis of the soldering procedure stage in which thepeel occurs. FIG. 15B shows a result of measurement of temperature ofthe portion to be soldered completely soldered by reflow solderingduring the flow soldering. FIG. 15C is a diagram showing that a heatinsulation tape for inhibiting heat conduction is attached to a surfaceto be flow-soldered opposite to a surface reflow-soldered with QFP ICbefore performing the flow soldering.

To perform the analysis, in the reflow soldering operation, thetemperature of the soldering terminal (lead terminal) of the portion tobe soldered or the solder was set to 230° C., and in the flow solderingoperation, the temperature of the solder was controlled to be 250° C.Moreover, in FIG. 15A, mark O indicates that the fillet peel does notoccur and mark x indicates that the fillet peel occurs.

As shown in FIG. 15A, even in any one of the tin-37 lead solder, tin-3silver-0.5 copper solder, and tin-3.5 silver-0.75 copper solder,regardless of the types of plating of the soldering terminal of thecomponent, that is, the tin-lead solder plating or tin-bismuth solderplating, it is found out that the fillet peel does not occur with onlythe reflow soldering operation of the first surface.

However, when the second surface is successively flow-soldered, it isseen that the fillet peel occurs only with the use of the tin-3silver-0.5 copper solder and tin-3.5 silver-0.75 copper solder duringthe reflow soldering of the first surface and with the component platedwith the tin-zinc solder. It is to be noted that the solder used in theflow soldering has substantially the same composition as that of thesolder used in the reflow soldering.

Further, as shown in FIG. 15C, it is seen that a heat insulation tape 40for suppressing temperature rise of the soldering terminal of QFP IC 16(240 pin QFP IC) is attached and the fillet peel does not occur even bythe flow soldering.

As shown in FIG. 15B, the temperature of the reflow-soldered portionduring the flow soldering is 182° C. in a usual soldered state withoutthe heat insulation tape 40, and rises to 122° C., when the heatinsulation tape 40 is attached. In this event, the temperatures of thefirst surface of the printed wiring board 10, namely, the surface of thereflow soldering surface were 201° C. and 133° C.

As confirmed from these analysis results, only in the soldered portionin which the electronic component 12 including the soldering terminalplated with the tin-zinc solder is reflow-soldered with thetin-silver-copper based lead-free solder, the fillet peel occurs by thesubsequently performed flow soldering. During the flow soldering, thetemperature of the soldering portion rises to 182° C. which exceeds themelting point of 178° C. of the tin-silver-lead ternary eutectic alloy,and this can be specified as the cause.

Next, the presence/absence of the fillet peel was checked by the use ofthe soldering method of the present invention. FIGS. 16A and 16B showtables showing the result of the check, and FIG. 16A is a tablecollectively showing the types of the solder for performing the reflowsoldering, types of plating of the electronic component, and temperatureof the tin-9 zinc solder for use in the flow soldering which arefactors, and the presence/absence of the fillet peel. FIG. 16B is atable comparing the temperatures of portions during the flow solderingin the type of the solder and appropriate solder temperatures for useduring the soldering.

Specifically, as shown in FIG. 16A, similarly as in the above-describedexample, the tin-3 silver-0.5 copper solder and tin-3.5 silver-0.75copper solder are used in the solder during the reflow soldering, thetin-9 zinc solder is used in the solder for performing the flowsoldering, and the temperatures are 220° C. and 230° C. This case wasstudied. As a result, in any case, the fillet peel did not occur.

Naturally, the tin-9 zinc solder sufficiently wets up also in the platedthrough hole, and also sufficiently wet-spreads over the land (stateshown in step (9) of FIG. 12). Moreover, when the printed wiring boardis HAL-treated by the tin-zinc solder, or when the acrylic acid adductof rosin and the activator including the sarcosine framework are used inthe flux composition, even with an extremely low temperature of thesolder of 220° C. to 230° C., a wetting-up speed of the tin-9 zincsolder is very high, and the fillet is extremely securely formed.

On the other hand, as shown in FIG. 16B, the temperature of the leadterminal portion of the 240 pin QFP IC in the flow soldering, that is,the soldering terminal is 151° C., when the temperature of the tin-9zinc solder is 220° C. The terminal temperature is 160° C., when thetemperature of the tin-9 zinc solder is 230° C. It is found out that thetemperature is far lower than the melting point of the tin-silver-leadternary eutectic alloy which is 178° C. In other words, when thetemperature of the soldering for the flow soldering can be reduced, thetemperature rise of the reflow-soldered portion is also reduced.

Additionally, in the flow soldering using the tin-37 lead solder, tin-3silver-0.5 copper solder, and tin-3.5 silver-0.75 copper solder at anappropriate flow soldering temperature of 250° C., the terminaltemperature is 182° C., as described above.

Specifically, with the flow soldering using the tin-9 zinc solder at acontrolled temperature of 220° C. to 230° C., the micro tin-silver-leadternary eutectic alloy present in the interface of the reflow-solderedportion is stopped from being molten, and the fillet peel can securelybe prevented.

It is to be noted that the printed wiring board used in the experimentas shown in FIGS. 15A through 15C and 16 is FR-4 material using a basematerial of most generally used 1.6 mm thick glass epoxy havingASTM/NEMA standard.

Third Embodiment

The flow soldering method of a third embodiment of the present inventioncan be performed as follows.

(1) Constitution of Flow Soldering Apparatus

At first, a constitution example of the flow soldering apparatus inwhich the flow soldering method of the present invention can beperformed will be described. FIG. 18 is a longitudinal sectional viewshowing one example of the flow soldering apparatus, and the nitrogengas supply system is shown in a symbol drawing. Specifically, the flowsoldering apparatus is constituted to perform the soldering operation inthe atmosphere of the inactive gas having the low oxygen concentration,namely, nitrogen gas. It is to be noted that the flow solderingapparatus of FIG. 18 has a constitution similar to that of the flowsoldering apparatus shown in FIGS. 8A and 8B and 14.

The conveyor for conveying the work to be soldered, that is, printedwiring board 401 is constituted of a first conveyor 402 forelevation-angle conveyance (elevation angle θ1) and second conveyor 403for depression-angle conveyance (depression angle θ2). A tunnel-shapedchamber 404 is disposed to cover these conveyors.

As shown in FIG. 18, the longitudinal section of the tunnel-shapedchamber 404 has an inverted V shape, and a conveying-in port 405 andconveying-out port 406 are constituted to have the same height from thehorizontal plane. Thereby, it is easy to retain the atmosphere in thechamber 404 at a temperature higher than that of the atmosphere outsidethe chamber 404, and the atmosphere in which components (e.g., oxygenconcentration) are stable can be formed.

The first and second conveyors 402, 403 include holding claws forholding opposite side ends of the printed wiring board 401, and eachconveyor includes two conveyor frames disposed on opposite side ends andin parallel with each other. Moreover, a preheater 407 constituting thepreliminary heating step of the printed wiring board 401, and a soldertank 408 constituting a soldering step are disposed along the firstconveyor 402 in the tunnel-shaped chamber 404. Herein, the shown arrow Aindicates the conveyance direction of the printed wiring board 401.

The solder tank 408 of the soldering step contains a tin-9 zinc solder409, and the solder 409 is heated by the heater, temperature sensor, andtemperature control apparatus (not shown) and has a molten state. Thetemperature is controlled at the predetermined temperature. This moltensolder 409 is sent to a first spray nozzle 411 by a first pump 410 toform a first jet flow 412. A second pump 413 sends the solder to asecond spray nozzle 414 to form a second jet flow 415.

Subsequently, these jet flows 412, 415 are brought into contact with thelower surface of the printed wiring board 401, namely, the surface to beflow-soldered of the lower surface of the printed wiring board 10 onwhich the portion to be soldered exists including the land 14 andthrough hole 11, and the soldering and lead terminals of the electroniccomponent 12 as shown in the step (7) of FIG. 12. Thereby, the moltensolder 409 is supplied to the portion to be soldered to solder theportion.

Moreover, the preheater 407 is disposed in the tunnel-shaped chamber404. However, the solder tank 408 is constituted such that an opening416 is disposed in the tunnel-shaped chamber 404 and the first andsecond jet flows 412 and 415 are positioned in the tunnel-shaped chamber404 through the opening 416. With this structure, a skirt 417 isdisposed in the opening 416. The skirt 417 is immersed in the moltensolder 409 in the solder tank 408 to realize the complete sealing.

Further, a large number of plate members, that is, inhibit plates 418are disposed along the longitudinal direction of the tunnel, that is,the conveyance direction of the conveyors 402, 403 in the tunnel-shapedchamber 404. The plates form the labyrinth seal, and are constituted toprevent an unnecessary atmospheric flow from being generated in thetunnel-shaped chamber 404.

Discharge ports 419 for supplying nitrogen gas which is the inactive gasinto the tunnel-shaped chamber 404 are disposed among the inhibit plates418 disposed after the solder tank 408 as viewed from the conveyancedirection, and the targeted nitrogen gas supply flow rate can beadjusted by flow rate adjustment valves 420 and flow rate meters 421.The nitrogen gas is supplied from a nitrogen gas supply apparatus 422 ofa bomb or PSA type, and supplied to the flow rate adjustment valves 420via an open/close valve 423, filter 424 for removing impurities, andpressure control valve 425 for adjusting a supply pressure to thetarget. A pressure meter 426 is disposed to monitor the pressure.

For the nitrogen gas supply flow rate, the oxygen concentration in thetunnel-shaped chamber 404 is measured by the oxygen concentration meter(not shown), for example, the atmosphere of the soldering step as thecontact region of the printed wiring board 401 with the jet flows 412,415 of the molten solder 409 is sampled, and the rate is measured. Theflow rate adjustment valves 420 are adjusted so as to obtain the targetoxygen concentration, and the flow rate is set.

If necessary, for the constitution as shown by a broken line, thedischarge port 419 for nitrogen gas supply is similarly disposed in thevicinity of the preheater 407 of the preliminary heating step, and theoxygen concentration of the atmosphere in the vicinity of the preheater407 may also be measured by the oxygen concentration meter. In thisconstitution, the oxygen concentrations of the preliminary heating stepand soldering step can also separately be adjusted/controlled.

The soldering operation is performed while the printed wiring board 401is conveyed by the conveyors 402, 403. Specifically, the printed wiringboard 401 supplied through the conveying-in port 405 is conveyed in thearrow A direction. The board is conveyed in order of the preliminaryheating step→soldering step, the molten solder 409 is supplied to theportion to be soldered of the printed wiring board 401 by the jet flows412, 415 in the soldering step to solder the portion, and the completelysoldered printed wiring board 401 is conveyed out via the conveying-outport 406.

The flow soldering operation is performed in the preliminary heatingstep, while the temperature of the portion to be soldered of the printedwiring board 401 indicates the predetermined target value in a range of80° C. to 150° C. The operation is performed in the soldering step,while the temperature of the tin-9 zinc solder indicates thepredetermined target value in a range of 220° C. to 230° C.

In one example, after the temperature of the surface to be soldered ofthe printed wiring board is preliminarily heated at about 120° C., thesurface is brought into contact with the jet flow of the tin-9 zincsolder at 220° C. to perform the flow soldering.

(2) Soldering Procedure

FIG. 17 is an explanatory view of a procedure for realizing the methodof flow soldering of the present invention, and FIGS. 17A to 17E arelongitudinal sectional views showing the states of the printed wiringboard and mounted electronic component for each procedure. Herein, theflow soldering apparatus of the above (1) is used in a step of FIG. 17E.

(Procedure 1)

With respect to a printed wiring board 170 including copper lands 171and copper through hole 172 constituting the portion to be soldered asshown in FIG. 17A, known HAL treatment is performed using thetin-silver-copper based solder (e.g., tin-3.5 silver-0.75 copper solder,tin-3 silver-0.5 copper solder), tin-silver based solder (e.g., tin-3.5silver solder), tin-copper based solder (e.g., tin-0.75 copper solder),or tin (100 tin). A layer 173 of the above-described solder or tin isformed on the surfaces of the copper lands 171 and copper through hole172 (copper plated through hole) (FIG. 17B). The layer of the solder ortin may also be used by other known methods such as plating.

In this case, a tin-rich lead-free solder is used in the solder.Specifically, the lead-free solder in which a content of tin is 90% ormore is preferable. As described later in examples, the wet-spread ofthe tin-zinc solder over the lands 171 or the wet-up of the tin-zincsolder in the plated through hole 172 are improved in satisfactorystates.

In this event, the tin-rich lead-free solder is limited to a soldercontaining no bismuth. When bismuth is contained, the solder of theportion to be soldered is embrittled after the soldering. Originalcharacteristics of the tin-zinc solder having high tenacity are lost andconnection strength of the soldered portion becomes weak against cyclestress.

(Procedure 2)

An electronic component 175 (e.g., SOP IC) is fixed and mounted onto theprinted wiring board 170 on which the layer of the solder or tin isdisposed as shown in FIG. 17B by an adhesive 174, as shown in FIG. 17C.Moreover, for the electronic component 175 including an insertion typelead terminal 176, as shown in FIG. 17D, after the printed wiring board170 is reversed as shown in FIG. 17D, a lead terminal 176 of thecorresponding electronic component 175 (e.g., DIP IC, SIP IC, ZIP IC) isinserted in the through hole 172 to mount the component. Naturally, theelectronic component 175 mounted in FIG. 17C or 17D is one example.

The terminal subjected to a lead-free metal plating or coating treatmentis used in the soldering terminal of the electronic component 175 orsoldering lead terminal 176. A metal for use in the plating or coatingtreatment differs with the type or manufacturer of the electroniccomponent 175. For passive components such as a resistance andcapacitor, the solder similar to that for use in the soldering is oftenused. However, wetting in die-bonding the lead frame is also consideredin semiconductor components such as IC, and palladium or palladium alloyis used as one example.

(Procedure 3)

When the mounting of the electronic component 175 onto the printedwiring board 170 is completed in the procedure 2, the printed wiringboard 170 is soldered by the flow soldering apparatus described in theabove (1). Specifically, the surface to be soldered shown in FIG. 17E(shown lower surface) is brought in contact with the jet flow of thetin-zinc solder (e.g., tin-9 zinc solder), the tin-zinc solder issupplied to wet a portion to be soldered 177, and the tin-zinc solderwets up in the plated through hole 172.

In this case, the layer of the tin-rich lead-free solder or tin isformed on the surfaces of the land and plated through hole of theprinted wiring board 170 by the procedure 1, and therefore thewet-spread of the tin-zinc solder in the land 171 and the wet-up in theplated through hole 172 are improved.

Also when the palladium or palladium alloy layer is disposed on thesurface of the soldering terminal of the electronic component 175mounted on the land 171 or the surface of the soldering lead terminal176 of the electronic component 175 inserted in the through hole 172,the tin-zinc solder well wets-spreads over the palladium or palladiumalloy layer. Therefore, superior soldering property is obtained. On theother hand, the superior wet-spread or wet-up of the tin-zinc soldercannot be obtained with another lead-free solder.

In the meantime, during the flow soldering of the printed wiring board,the post flux is usually applied before the preliminary heating step toprevent oxidation and improve wettability. However, when the fluxcontaining acrylic acid adduct of rosin and the activator including thesarcosine framework is used, the wettability is persistently enhanced incontact with the jet flow. More satisfactory wettability is obtainedparticularly in the tin-zinc solder.

Third Embodiment

(1) Evaluation of Soldering Property by Spread Ratio Method

A spread ratio test of the solder was performed in conformity toJIS-Z-3184, and the soldering property was evaluated.

First, a phosphor deoxidized copper plate (JIS C-2100 50×50×0.3 mm) wasplated (leveler treatment) with molten tin (100 tin), tin-0.75 coppersolder, tin-3 silver-0.5 copper solder, tin-3.5 silver solder, andtin-3.5 silver-0.75 copper solder to prepare test pieces three for each.Three test pieces of the phosphor deoxidized copper plate (100 copper)which were not plated/treated were prepared.

Next, a tin-9 zinc solder paste was printed on each test piece.Subsequently, the test piece was introduced into a reflow furnace inwhich nitrogen atmosphere having oxygen concentration of 500 ppm wasformed, and soldered on heating conditions that peak temperature was220° C. and holding time was 60 seconds at 200° C. or more.

Then, each soldered test piece was cooled at room temperature.Thereafter, the flux remaining on the surface of each test piece wascleaned/removed with alcohol. Successively, the height of the solder oneach test piece was measured, and each spread ratio S(%) was obtained inthe following equation (1).S=(D−H)/D×100(%)  (1)Herein, H denotes a height (mm) of the spread solder, and D denotes adiameter (mm) of a volume of the printed solder which is regarded as aball.D=1.24 V^(1/3)  (2)V=mass/specific gravity of printed solder  (3)Here, meanings of H and D are illustrated in FIG. 19.

These measurement results are collectively shown in a table in adiagram/graph of FIGS. 20A and 20B, respectively. FIG. 20A is adiagram/table showing the spread state of the tin-zinc solder in photosand each center value of the spread ratio S(%). FIG. 20B is a diagramshowing the spread ratio S(%) including a deviation. It is to be notedthat each element is shown by an element symbol in FIGS. 20A and 20B.

Specifically, with 100 copper corresponding to the printed wiring boardon which the copper land and through hole are formed, the wet-spreadratio of the tin-9 zinc solder is 82.3%. In contrast, when the moltenplating treatment corresponding to the leveler treated printed wiringboard is tin (100 tin), the ratio is 90.9%. The wet-spread ratio is91.4% with the tin-0.75 copper solder, 90.3% with the tin-3 silver-0.5copper, 89.4% with the tin-3.5 silver solder, and 89.3% with the tin-3.5silver-0.75 copper solder.

It is seen from FIG. 20B that with more content of tin of the levelertreatment, the wet-spread of the tin-zinc solder is proportionallyimproved.

Further, it can be concluded from this diagram/table that the levelertreatment is performed by tin or the tin-rich solder containing acontent of tin of 90 weight % or more, and the satisfactory wet-spreadratio of about 85% or more can be obtained.

(2) Evaluation by Flow Soldering

Next, the land to be soldered and plated through hole of the printedwiring board were variously leveler-treated, the printed wiring board onwhich the layer of the tin or tin-rich solder was formed wasflow-soldered by the tin-9 zinc solder, and soldering property wasevaluated. Upon the soldering, the temperature of the solder or jet flowis low at 220° C.

FIGS. 21A and 21B show a table and a diagram, respectively, showing theresults of the flow soldering by the tin-zinc solder of the printedwiring board subjected to various leveler treatments. FIG. 21A is adiagram (the element symbol is used to show each element) showingsoldering results in a table. FIG. 21B is a vertical section of theportion to be soldered showing an evaluation standard of the solderingproperty. It is to be noted that the evaluation standard in FIG. 21B isshown for the lead terminal of the electronic component inserted in theplated through hole with most difficulty in the flow soldering.

Specifically, referring to FIGS. 21A and 21B, in the printed wiringboard, a layer of tin (100 tin), tin-1.2 silver-0.75 copper solder,tin-3 silver-0.5 copper solder, tin-3.5 silver-0.75 copper solder, ortin-9 zinc solder is formed on the land to be soldered and platedthrough hole of the printed wiring board. Also in the printed wiringboard, the copper land or through hole is plated with nickel and thisfoundation is plated with gold. Moreover, the copper land or throughhole is not treated in a copper printed wiring board. With respect tothe respective boards, the soldering property by the tin-9 zinc solderis evaluated and the result is shown.

To perform this evaluation, the electronic component in which the layerof the tin-zinc solder is formed on the soldering lead terminal isinserted in the through hole and mounted. The electronic component isinserted in the through hole and mounted, in which soldering leadterminal is plated with nickel, this underlayer is plated withpalladium, and the palladium layer is formed on the surface of thesoldering lead terminal. Further, the used electronic component is DIPIC.

In this event, the electronic component including the soldering leadterminal with the layer of the tin-zinc solder formed thereon isinserted in the through hole and mounted and this is evaluated tocompare this with a related-art example using lead.

Next, the evaluation standard shown in FIG. 21B will be described. Asshown, “O” indicates a satisfactory soldered state in which the solderwets up in the through hole and the solder fillets are formed over thelands on the lower and upper surfaces of the printed wiring board. Thesolder fillet is formed on the land of the lower surface of the printedwiring board and the solder also wets up in the through hole. However,the solder cannot wet-spread over the upper land of the printed wiringboard and the fillet is not formed. This state is indicated as theslightly insufficient soldered state by “Δ.” The solder fillet is formedon the land of the lower surface of the printed wiring board, but thesolder does not sufficiently wet up in the through hole. Therefore, thesolder cannot wet-spread over the upper land of the printed wiring boardand the fillet is not formed. This state is shown as the insufficientsoldered state by “x.”

From the evaluation result of FIG. 21A, the electronic component mountedon the printed wiring board and including the soldering lead terminalwith the lead-free palladium layer formed thereon has an insufficientsoldered state with the printed wiring board plated with nickel/gold orthe printed wiring board which is not treated. However, in the printedwiring board in which the tin layer or the layer of the tin-rich solderis formed, satisfactory soldered state is obtained.

The electronic component (particularly, IC including the lead frame)including the palladium or palladium alloy layer formed on the surfaceof the soldering terminal or lead terminal is generally assumed to beinferior to that including the layer of the tin-lead solder inwettability to the lead-free solder. However, according to the flowsoldering method of the present invention, the satisfactory solderedstate can be obtained.

In the printed wiring board subjected to the nickel/gold platingtreatment, superior wettability and soldering property are obtained,when the related-art layer of the tin-zinc solder is formed on theelectronic component. However, it is seen that the insufficient solderedstate is obtained with the palladium layer formed on the soldering leadterminal of the electronic component.

Further, the tin-zinc solder was raised at 230° C. and 250° C. toperform the flow soldering, but the similar evaluation result isobtained. A superiority difference by the temperature of the solder andjet flow was not recognized.

In this manner, the layer of the lead-free solder of tin (100 tin) orrich in tin (tin content of 90 weight % or more) is formed beforehand onthe surfaces of the land to be soldered and plated through hole of theprinted wiring board. The electronic component is mounted on the printedwiring board and flow-soldered by the tin-zinc solder, so thatsatisfactory soldering property is obtained.

Specifically, when the tin-zinc solder is used to perform the flowsoldering, main factor of the printed wiring board concerning thewettability is controlled. Even with the use of the tin-zinc solderwhich has heretofore been evaluated as bad in the wettability and whichhas not been an object solder of the flow soldering, it is possible tostably produce printed wiring boards in mass with superior solderingquality.

Additionally, even with the solder having a low temperature of thesolder and jet flow at 220° C., satisfactory wet-spread and wet-up areobtained in the land to be soldered and plated through hole. Thistemperature is much low as compared with the temperature (about 240° C.to 250° C.) during the flow soldering by the related-art tin-lead solderor temperature (about 250° C. to 255° C.) during the flow soldering by ahigh melting point lead-free solder.

As a result, heat stress imposed onto the electronic component isreduced during the soldering, and the life of the electronic componentin which this heat stress is a factor is much longer than that of therelated-art tin-lead solder or high-melting point lead-free solder. Onlydeterioration of the electronic component by another factor may be aproblem.

Further, it is known that the tin-zinc solder has strong tenacity evenas compared with any other lead-free solder, and has appropriatehardness and superior elongation characteristic. Therefore, it ispossible to maintain the connection state to be tenacious against thecycle stress. Therefore, with the use in an apparatus (e.g., car) havingmuch heat or acceleration cycle stress, trouble occurrence because ofconnection breakage (disconnection) of the portion to be soldered isextremely little, and outstanding reliability can be obtained.

(3) Flux which can Obtain Persistent Function

For the evaluation by the flow soldering, as usual, the post flux wassprayed/applied to the printed wiring board. In addition to gum rosinwhich is the related-art resin component of the post flux, the acrylicacid adduct of rosin having high heat resistance is selectively used inthe flux. Further, from a viewpoint of securing persistent wettabilityenhancement, the activator including the sarcosine framework isselectively used in the activator to be blended. The acrylic acid adductof rosin includes an addition reactant of acrylic acid and rosin orrosin derivative further subjected to hydrogenation reaction. Examplesof rosin include gum rosin, wood rosin, and tall oil rosin containingresin acids such as abietic acid, palustric acid, neoabietic acid,pimaric acid, isopimaric acid, and dehydroabietic acid as maincomponents. Moreover, examples of the activator including the sarcosineframework include oleyl sarcosine, lauroyl sarcosine, and palm oil fattyacid sarcosine.

In addition to the above-described specific constituting components,various components for use in the conventional flux can also be usedsuch as resin, activator, solvent, antioxidant, delusterant, and otheradditives.

Examples of the resin component for use in the conventional flux includevarious rosin derivatives such as gum rosin, polymerized rosin,deproportionated rosin, and hydrogenated rosin, and synthetic resinssuch as a polyamide resin and terpene resin.

Moreover, examples of the activator for use in the conventional fluxinclude hydrohalide of amine, organic acids, organic amine, and organichalide. Concrete examples of hydrohalide of amine include diethylaminehydrochloride, and cyclohexylamine hydrobromide. Concrete examples oforganic acids include adipic acid, stearic acid, and benzoic acid.Concrete examples of organic amines include hexylamine, dibutylamine,and triethylamine. Concrete examples of organic halides include2,3-dibromo-1-propanol, and 2,3-dibromo-2-butene-1,4-diol. Further,examples of the solvent include isopropyl alcohol, acetic ether, andtoluene.

Moreover, antioxidant, delusterant, and other additives are notespecially limited, and various known agents may appropriately beselected and used.

Next, the present invention will be described hereinafter in more detailin terms of examples of the flux. The liquid flux composition includingExamples 5 to 8 and Comparative Examples 4 to 6 was prepared, andspray-applied to the printed wiring board on which the electroniccomponent was mounted, and the component was soldered by theabove-described flow soldering apparatus.

FIG. 22A is a diagram/table showing the evaluation results usingexamples. As shown in FIG. 22B, the evaluation results OΔx wereevaluated by the same evaluation standard as that shown in FIG. 21B.(Example 5) Adipic acid 1.0 wt % (weight %) Dibutyl amine 1.0 wt %Cyclohexylamine hydrobromide 0.2 wt % Oleyl sarcosine 1.0 wt % Acrylicacid adduct of rosin 7.0 wt % Hydrogenerated rosin 7.0 wt % Isopropylalcohol 82.8 wt % (Example 6) Sebacic acid 1.0 wt %2,3-dibromo-1-propanol 0.5 wt % Diethylamine hydrochloride 0.2 wt %Lauroyl sarcosine 1.0 wt % Acrylic acid adduct of rosin 10.0 wt % Gumrosin 5.0 wt % Isopropyl alcohol 82.3 wt % (Example 7) Octyl acid 1.0 wt% Benzoic acid 1.0 wt % Diphenylguanidine hydrobromide 0.2 wt % Palm oilfatty acid sarcosine 1.0 wt % Acrylic acid adduct of rosin 20.0 wt %Isopropyl alcohol 77.8 wt % (Example 8) Glutaric acid 1.0 wt %2,3-dibromo-1-propanol 0.5 wt % Cyclohexylamine hydrobromide 0.2 wt %Oleyl sarcosine 1.0 wt % Acrylic acid adduct of rosin 7.0 wt %Deproportionated rosin 7.0 wt % Isopropyl alcohol 83.3 wt % (ComparativeExample 4) Adipic acid 1.0 wt % Dibutyl amine 1.0 wt % Cyclohexylaminehydrobromide 0.2 wt % Gum rosin 15.0 wt % Isopropyl alcohol 82.8 wt %(Comparative Example 5) Sebacic acid 1.0 wt % 2,3-dibromo-1-propanol 0.5wt % Diethylamine hydrochloride 0.2 wt % Lauroyl sarcosine 1.0 wt % Gumrosin 15.0 wt % Isopropyl alcohol 82.3 wt % (Comparative Example 6)Octyl acid 1.0 wt % Benzoic acid 1.0 wt % Diphenylguanidine hydrobromide0.2 wt % Acrylic acid adduct of rosin 10.0 wt % Hydrogenerated rosin10.0 wt % Isopropyl alcohol 77.8 wt %

As shown in the evaluation results of FIGS. 22A and 22B, satisfactorysoldering properties are obtained in Examples 5, 6, 7, 8.

In this manner, by the flux composition of the present embodiment whichis superior in heat resistance and in which wettability enhancement isobtained, the function of the flux such as the enhancement ofwettability in the contact of the printed wiring board with the jet flowof the tin-zinc solder is persistently obtained. When the tin-zincsolder is wet-spread over the land to be soldered, or wetted up in theplated through hole, preferable function is obtained.

According to the soldering method of the first aspect of the presentinvention, it is possible to perform soldering/mounting withoutgenerating the lift-off. To solder the electronic component includingthe palladium or palladium alloy layer on the soldering lead terminalonto the printed wiring board, it is possible to perform the printedwiring board which is superior in solder wettability and whosemechanical connection is tenacious and whose electric connection issatisfactory and which has high soldering reliability.

Moreover, according to the solder joint member of the first aspect ofthe present invention, it is possible to establish both high reliabilitycharacteristic of the so-called lead-free semiconductor component whichuses the lead frame with the palladium or palladium alloy layer formedthereon and which does not use lead, and high reliability characteristicin the soldering/mounting of the semiconductor component onto theprinted wiring board.

Thereby, in all steps from the packaging of the semiconductor componentto the soldering/mounting of the semiconductor component onto theprinted wiring board, the high reliability characteristic can beobtained. As a result, the electronic component extremely high inreliability can be obtained.

According to the second aspect of the present invention, the electroniccomponent having a condition that the land peel occurs during the flowsoldering or the lead-free solder (soldering terminal of the tin-leadsolder plating and tin-silver-copper based lead-free solder) is used toreflow-solder the printed wiring board. Thereafter, the surface oppositeto the reflow-soldered surface is flow-soldered using the tin-9 zincsolder in a controlled temperature range of 220° C. to 230° C. Thereby,it is possible to hold the temperature of the reflow-soldered portion at178° C. or less which is the melting point of the tin-silver-leadternary eutectic alloy, and the land peel is prevented from occurringduring the flow soldering.

Moreover, since the solder does not contain bismuth, the solderingstrength is tenacious and the soldering strong against the cycle stresscan be performed.

Further, with the use of the printed wiring board HAL-treated by thetin-zinc solder, although the temperature of the tin-9 zinc solder foruse in the flow soldering is low, the tin-9 zinc solder rapidlywet-spreads over the plated through hole and connected land, and thesecure and satisfactory soldering can be performed even in the throughhole.

Additionally, the flux well matched to the tin-zinc solder which iseasily oxidized is used, and thereby the soldering can be performed withfurther superior solder wettability.

As a result, the land peel with the use of the lead-free solder issecurely blocked, and it is possible to solder/mount the printed wiringboard superior in soldering quality and reliability.

According to the flow soldering method of the third aspect of thepresent invention, in the flow soldering operation using the tin-zincsolder, major factors influencing the soldering quality are controlled.Particularly, the major factors in the printed wiring board arecontrolled. The satisfactory wet-spread and wet-up of the tin-zincsolder can be obtained. To solder/mount a large number of variouselectronic components onto the printed wiring board using the tin-zincsolder, each portion to be soldered can uniformly and steadily besoldered in the superior soldered state.

Additionally, it is possible to perform the flow soldering with thesolder having low temperature at 220° C., and it is possible to lengthenthe life of the electronic component mounted on the printed wiring boardand the life of the electronic apparatus on which the printed wiringboard is mounted.

Therefore, it is possible to realize the electronic apparatus which isstrong against the cycle stress and which has long life, that is, thehigh-reliability electronic apparatus. Further, since zinc isinexpensive, the apparatus can be manufactured at a cost lower than thatof another lead-free solder.

While the present invention has thus far been disclosed in conjunctionwith several embodiments thereof, it will be readily possible for thoseskilled in the art to put the present invention into practice in variousother manners.

1. A method for soldering a printed wiring board including portions tobe soldered on opposite surfaces, comprising the steps of:reflow-soldering an electronic component to one surface of the printedwiring board by a lead-free solder containing at least tin, silver, andcopper; and subsequently controlling the solder of the portion to besoldered formed on the other surface of the printed wiring board by atin-9 zinc solder in a temperature range of 220° C. to 230° C.; andholding the temperature of the reflow-soldered portion to be soldered at178° C. or less to perform flow soldering.
 2. The method according toclaim 1, wherein: the electronic component includes a soldering terminalcoated with a tin-zinc solder.
 3. The method according to claim 1,further comprising: forming a plated through hole in the printed wiringboard; and HAL-treating the printed wiring board by a tin-zinc solder.4. The method according to claim 1, wherein: a flux for use in the flowsoldering contains an acrylic acid adduct of rosin and an activatorincluding a sarcosine framework.
 5. The method according to claim 1,wherein: the solder does not contain bismuth.
 6. A method for solderinga printed wiring board including portions to be soldered on oppositesurfaces, comprising the steps of: reflow-soldering an electroniccomponent to one surface of the printed wiring board by a lead-freesolder containing at least tin, silver, and copper; subsequentlyflow-soldering the portion to be soldered formed on the other surface ofthe printed wiring board by a tin-9 zinc solder; and performing the flowsoldering so that temperature of the reflow-soldered portion does notexceed a melting point of a ternary eutectic alloy of tin-silver-lead.7. The method according to claim 6, wherein: the electronic componentincludes a soldering terminal coated with a tin-zinc solder.
 8. Thesoldering method according to claim 6, further comprising: forming aplated through hole in the printed wiring board; and HAL-treating theprinted wiring board by a tin-zinc solder.
 9. The method according toclaim 6, wherein: a flux for use in the flow soldering contains anacrylic acid adduct of rosin and an activator including a sarcosineframework.
 10. The method according to claim 6, wherein: the solder doesnot contain bismuth.